Doc: continuation

doc/Expr.adoc

@@ -79,20 +79,20 @@ Here are some examples of execution.
 # Type 'exit' or Ctrl+D to quit the program.
 
 [user]$ ./dev-expr
-dev-expr -- Expressions calculator v1.10.0(build 14),2024/06/17 (celestino.amoroso@portale-stac.it)
-	Based on the Expr package v0.19.0
+dev-expr -- Expressions calculator v1.12.0(build 1),2024/09/14 (celestino.amoroso@portale-stac.it)
+	Based on the Expr package v0.26.0
 	Type help to get the list of available commands
 	See also https://git.portale-stac.it/go-pkg/expr/src/branch/main/README.adoc
 >>> help
 --- REPL commands:
-      source -- Load a file as input
-         tty -- Enable/Disable ansi output <1>
         base -- Set the integer output base: 2, 8, 10, or 16
         exit -- Exit the program
         help -- Show command list
           ml -- Enable/Disable multi-line output
         mods -- List builtin modules
       output -- Enable/Disable printing expression results. Options 'on', 'off', 'status'
+      source -- Load a file as input
+         tty -- Enable/Disable ansi output <1>
 
 --- Command line options:
     -b <builtin>       Import builtin modules.
@@ -155,9 +155,9 @@ _Expr_ supports three type of numbers:
 
 . [blue]#Integers#
 . [blue]#Floats#
-. [blue]#Factions#
+. [blue]#Fractions#
 
-In mixed operations involving integers, fractions and floats, automatic type promotion to the largest type take place.
+In mixed operations involving integers, fractions and floats, automatic type promotion to the largest type is performed.
 
 ==== Integers
 __Expr__'s integers are a subset of the integer set. Internally they are stored as Golang _int64_ values.
@@ -183,11 +183,11 @@ Value range: *-9223372036854775808* to *9223372036854775807*
 [cols="^1,^2,6,4"]
 |===
 | Symbol    | Operation     | Description               | Examples
-| [blue]`+` | _sum_         | Add two values            | [blue]`-1 + 2` -> 1
-| [blue]`-` | _subtraction_ | Subtract the right value from the left one | [blue]`3 - 1` -> 2
-| [blue]`*` | _product_     | Multiply two values       | [blue]`-1 * 2` -> -2
-| [blue]`/` | _Division_    | Divide the left value by the right one^(*)^ | [blue]`-10 / 2` -> 5
-| [blue]`%` | _Modulo_      | Remainder of the integer division | [blue]`5 % 2` -> 1
+| [blue]`+` | _Sum_         | Add two values            | [blue]`-1 + 2` -> _1_
+| [blue]`-` | _Subtraction_ | Subtract the right value from the left one | [blue]`3 - 1` -> _2_
+| [blue]`*` | _Product_     | Multiply two values       | [blue]`-1 * 2` -> _-2_
+| [blue]`/` | _Integer division_    | Divide the left value by the right one^(*)^ | [blue]`-11 / 2` -> _-5_
+| [blue]`%` | _Modulo_      | Remainder of the integer division | [blue]`5 % 2` -> _1_
 |===
 
 ^(*)^ See also the _float division_ [blue]`./` below.
@@ -228,11 +228,11 @@ _dec-seq_ = _see-integer-literal-syntax_
 [cols="^1,^2,6,4"]
 |===
 | Symbol    | Operation     | Description               | Examples
-| [blue]`+` | _sum_         | Add two values            | [blue]`4 + 0.5` -> 4.5
-| [blue]`-` | _subtraction_ | Subtract the right value from the left one | [blue]`4 - 0.5` -> 3.5
-| [blue]`*` | _product_     | Multiply two values       | [blue]`4 * 0.5` -> 2.0
-| [blue]`/` | _Division_    | Divide the left value by the right one | [blue]`1.0 / 2` -> 0.5
-| [blue]`./`| _Float division_ | Force float division   | [blue]`-1 ./ 2` -> -0.5
+| [blue]`+` | _Sum_         | Add two values            | [blue]`4 + 0.5` -> 4.5
+| [blue]`-` | _Subtraction_ | Subtract the right value from the left one | [blue]`4 - 0.5` -> 3.5
+| [blue]`*` | _Product_     | Multiply two values       | [blue]`4 * 0.5` -> 2.0
+| [blue]`/` | _Float division_    | Divide the left value by the right one | [blue]`1.0 / 2` -> 0.5
+| [blue]`./`| _Forced float division_ | Force float division   | [blue]`-1 ./ 2` -> -0.5
 |===
 
 ==== Fractions
@@ -308,7 +308,7 @@ Strings are character sequences enclosed between two double quote [blue]`"`.
 `>>>` [blue]`"123\tabc"` +
 [green]`123{nbsp}{nbsp}{nbsp}{nbsp}abc`
 
-Some arithmetic operators can also be used with strings.
+Some arithmetic operators also apply to strings.
 
 .String operators
 [cols="^1,^2,6,4"]
@@ -444,6 +444,7 @@ _non-empty-list_ = "**[**" _any-value_ {"**,**" _any-value_} "**]**" +
 | [blue]`[]` | _Item at index_ | Item at given position | [blue]`[1,2,3][1]` -> _2_
 | [blue]`in` | _Item in list_ | True if item is in list | [blue]`2 in [1,2,3]` -> _true_ +
 [blue]`6 in [1,2,3]` -> _false_
+| [blue]`#` | _Size_ | Number of items in a list | [blue]`#[1,2,3]` -> _3_
 |===
 
 Array's items can be accessed using the index `[]` operator.
@@ -458,10 +459,16 @@ Array's items can be accessed using the index `[]` operator.
 *_slice_* = _string-expr_ "**[**" _integer-expr_ "**:**" _integer-expr_ "**]**"
 ====
 
-.Items of list
+.Examples: Getting items from lists
 `>>>` [blue]`[1,2,3][1]` +
 [green]`2`
 
+`>>>` [blue]`index=2; ["a", "b", "c", "d"][index]` +
+[green]`c`
+
+`>>>` [blue]`["a", "b", "c", "d"][2:]` +
+[green]`["c", "d"]`
+
 `>>>` [blue]`list=[1,2,3]; list[1]` +
 [green]`2`
 
@@ -471,25 +478,44 @@ Array's items can be accessed using the index `[]` operator.
 `>>>` [blue]`list=["one","two","three"]; list[2-1]` +
 [green]`two`
 
+`>>>` [blue]`list[1]="six"; list` +
+[green]`["one", "six", "three"]`
+
 `>>>` [blue]`list[-1]` +
 [green]`three`
 
 `>>>` [blue]`list[10]` +
 [red]`Eval Error: [1:9] index 10 out of bounds`
 
+.Example: Number of elements in a list
 `>>>` [blue]`#list` +
 [green]`3`
 
-`>>>` [blue]`index=2; ["a", "b", "c", "d"][index]` +
-[green]`c`
+.Examples: Element insertion
+`>>>` [blue]`"first" >> list` +
+[green]`["first", "one", "six", "three"]`
 
+`>>>` [blue]`list << "last"` +
+[green]`["first", "one", "six", "three", "last"]`
+
+.Examples: Element in list
+`>>>` [blue]`"six" in list` +
+[green]`true`
+
+`>>>` [blue]`"ten" in list` +
+[green]`false`
+
+.Examples: Concatenation and filtering
+`>>>` [blue]`[1,2,3] + ["one", "two", "three"]` +
+[green]`[1, 2, 3, "one", "two", "three"]`
+
+`>>>` [blue]`[1,2,3,4] - [2,4]` +
+[green]`[1, 3]`
 
-`>>>` [blue]`["a", "b", "c", "d"][2:]` +
-[green]`["c", "d"]`
 
 
 === Dictionaries
-The _dictionary_, or _dict_, data-type is set of pairs _key/value_. It is also known as _map_ or _associative array_.
+The _dictionary_, or _dict_, data-type represents sets  of pairs _key/value_. It is also known as _map_ or _associative array_.
 
 Dictionary literals are sequences of pairs separated by comma [blue]`,` enclosed between brace brackets.
 
@@ -515,6 +541,7 @@ _non-empty-dict_ = "**{**" _key-scalar_ "**:**" _any-value_ {"**,**" _key-scalar
 | [blue]`[]` | _Dict item value_ | Item value of given key | [blue]`{"one":1, "two":2}["two"]` -> _2_
 | [blue]`in` | _Key in dict_ | True if key is in dict | [blue]`"one" in {"one":1, "two":2}` -> _true_ +
 [blue]`"six" in {"one":1, "two":2}` -> _false_
+| [blue]`#` | _Size_ | Number of items in a dict | [blue]`#{1:"a",2:"b",3:"c"}` -> _3_
 |===
 
 .Examples
@@ -533,6 +560,9 @@ _non-empty-dict_ = "**{**" _key-scalar_ "**:**" _any-value_ {"**,**" _key-scalar
 `>>>` [blue]`d={"one":1, "two":2}; d["six"]=6; d` +
 [green]`{"two": 2, "one": 1, "six": 6}`
 
+`>>>` [blue]`#d` +
+[green]`3`
+
 
 == Variables
 _Expr_, like most programming languages, supports variables. A variable is an identifier with an assigned value. Variables are stored in _contexts_.
@@ -556,7 +586,7 @@ NOTE: The assign operator [blue]`=` returns the value assigned to the variable.
 `>>>` [blue]`a_b` +
 [green]`3`
 
-`>>>` [blue]`x = 5.2 * (9-3)`  [gray]_// The assigned value has the typical approximation error of the float data-type_ +
+`>>>` [blue]`x = 5.2 * (9-3)`  [gray]_// The assigned value here has the typical approximation error of the float data-type_ +
 [green]`31.200000000000003`
 
 `>>>` [blue]`x = 1; y = 2*x` +
@@ -664,8 +694,8 @@ The [blue]`:` symbol (colon) is the separator of the selector-cases. Note that i
 [red]`Eval Error: [1:3] no case catches the value (10) of the selection expression`
 
 
-=== Variable default value [blue]`??` and [blue]`?=`
-The left operand of these two operators must be a variable. The right operator can be any expression. They return the value of the variable if this is defined; otherwise they return the value of the right expression.
+=== Variable default value [blue]`??`, [blue]`?=`, and [blue]`?!`
+The left operand of first two operators, [blue]`??` and [blue]`?=`, must be a variable. The right operator can be any expression. They return the value of the variable if this is defined; otherwise they return the value of the right expression.
 
 IMPORTANT: If the left variable is defined, the right expression is not evaluated at all.
 
@@ -673,6 +703,10 @@ The [blue]`??` operator do not change the status of the left variable.
 
 The [blue]`?=` assigns the calculated value of the right expression to the left variable.
 
+The third one, [blue]`?!`, is the alternate operator. If the variable on the left size is not defined, it returns [blue]_nil_. Otherwise it returns the result of the expressione on the right side.
+
+IMPORTANT: If the left variable is NOT defined, the right expression is not evaluated at all.
+
 .Examples
 `>>>` [blue]`var ?? (1+2)` +
 [green]`3`

doc/Expr.html

@@ -567,7 +567,7 @@ pre.rouge .ss {
 <li><a href="#_but_operator">4.2. <code class="blue">but</code> operator</a></li>
 <li><a href="#_assignment_operator">4.3. Assignment operator <code class="blue">=</code></a></li>
 <li><a href="#_selector_operator">4.4. Selector operator <code class="blue">? : ::</code></a></li>
-<li><a href="#_variable_default_value_and">4.5. Variable default value <code class="blue">??</code> and <code class="blue">?=</code></a></li>
+<li><a href="#_variable_default_value_and">4.5. Variable default value <code class="blue">??</code>, <code class="blue">?=</code>, and <code class="blue">?!</code></a></li>
 </ul>
 </li>
 <li><a href="#_priorities_of_operators">5. Priorities of operators</a></li>
@@ -687,20 +687,20 @@ pre.rouge .ss {
 <pre class="rouge highlight"><code data-lang="shell"><span class="c"># Type 'exit' or Ctrl+D to quit the program.</span>
 
 <span class="o">[</span>user]<span class="nv">$ </span>./dev-expr
-dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o">(</span>build 14<span class="o">)</span>,2024/06/17 <span class="o">(</span>celestino.amoroso@portale-stac.it<span class="o">)</span>
-	Based on the Expr package v0.19.0
+dev-expr <span class="nt">--</span> Expressions calculator v1.12.0<span class="o">(</span>build 1<span class="o">)</span>,2024/09/14 <span class="o">(</span>celestino.amoroso@portale-stac.it<span class="o">)</span>
+	Based on the Expr package v0.26.0
 	Type <span class="nb">help </span>to get the list of available commands
 	See also https://git.portale-stac.it/go-pkg/expr/src/branch/main/README.adoc
 <span class="o">&gt;&gt;&gt;</span> <span class="nb">help</span>
 <span class="nt">---</span> REPL commands:
-      <span class="nb">source</span> <span class="nt">--</span> Load a file as input
-         <span class="nb">tty</span> <span class="nt">--</span> Enable/Disable ansi output <i class="conum" data-value="1"></i><b>(1)</b>
         base <span class="nt">--</span> Set the integer output base: 2, 8, 10, or 16
         <span class="nb">exit</span> <span class="nt">--</span> Exit the program
         <span class="nb">help</span> <span class="nt">--</span> Show <span class="nb">command </span>list
           ml <span class="nt">--</span> Enable/Disable multi-line output
         mods <span class="nt">--</span> List <span class="nb">builtin </span>modules
       output <span class="nt">--</span> Enable/Disable printing expression results. Options <span class="s1">'on'</span>, <span class="s1">'off'</span>, <span class="s1">'status'</span>
+      <span class="nb">source</span> <span class="nt">--</span> Load a file as input
+         <span class="nb">tty</span> <span class="nt">--</span> Enable/Disable ansi output <i class="conum" data-value="1"></i><b>(1)</b>
 
 <span class="nt">---</span> Command line options:
     <span class="nt">-b</span> &lt;<span class="nb">builtin</span><span class="o">&gt;</span>       Import <span class="nb">builtin </span>modules.
@@ -805,12 +805,12 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 <p><span class="blue">Floats</span></p>
 </li>
 <li>
-<p><span class="blue">Factions</span></p>
+<p><span class="blue">Fractions</span></p>
 </li>
 </ol>
 </div>
 <div class="paragraph">
-<p>In mixed operations involving integers, fractions and floats, automatic type promotion to the largest type take place.</p>
+<p>In mixed operations involving integers, fractions and floats, automatic type promotion to the largest type is performed.</p>
 </div>
 <div class="sect3">
 <h4 id="_integers"><a class="anchor" href="#_integers"></a><a class="link" href="#_integers">2.1.1. Integers</a></h4>
@@ -855,33 +855,33 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">+</code></p></td>
-<td class="tableblock halign-center valign-top"><p class="tableblock"><em>sum</em></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Sum</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Add two values</p></td>
-<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">-1 + 2</code> &#8594; 1</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">-1 + 2</code> &#8594; <em>1</em></p></td>
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">-</code></p></td>
-<td class="tableblock halign-center valign-top"><p class="tableblock"><em>subtraction</em></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Subtraction</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Subtract the right value from the left one</p></td>
-<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">3 - 1</code> &#8594; 2</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">3 - 1</code> &#8594; <em>2</em></p></td>
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">*</code></p></td>
-<td class="tableblock halign-center valign-top"><p class="tableblock"><em>product</em></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Product</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Multiply two values</p></td>
-<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">-1 * 2</code> &#8594; -2</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">-1 * 2</code> &#8594; <em>-2</em></p></td>
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">/</code></p></td>
-<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Division</em></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Integer division</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Divide the left value by the right one<sup>(*)</sup></p></td>
-<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">-10 / 2</code> &#8594; 5</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">-11 / 2</code> &#8594; <em>-5</em></p></td>
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">%</code></p></td>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><em>Modulo</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Remainder of the integer division</p></td>
-<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">5 % 2</code> &#8594; 1</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">5 % 2</code> &#8594; <em>1</em></p></td>
 </tr>
 </tbody>
 </table>
@@ -946,31 +946,31 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">+</code></p></td>
-<td class="tableblock halign-center valign-top"><p class="tableblock"><em>sum</em></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Sum</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Add two values</p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">4 + 0.5</code> &#8594; 4.5</p></td>
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">-</code></p></td>
-<td class="tableblock halign-center valign-top"><p class="tableblock"><em>subtraction</em></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Subtraction</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Subtract the right value from the left one</p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">4 - 0.5</code> &#8594; 3.5</p></td>
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">*</code></p></td>
-<td class="tableblock halign-center valign-top"><p class="tableblock"><em>product</em></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Product</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Multiply two values</p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">4 * 0.5</code> &#8594; 2.0</p></td>
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">/</code></p></td>
-<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Division</em></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Float division</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Divide the left value by the right one</p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">1.0 / 2</code> &#8594; 0.5</p></td>
 </tr>
 <tr>
 <td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">./</code></p></td>
-<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Float division</em></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Forced float division</em></p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock">Force float division</p></td>
 <td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">-1 ./ 2</code> &#8594; -0.5</p></td>
 </tr>
@@ -1074,7 +1074,7 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 <code class="green">123&#160;&#160;&#160;&#160;abc</code></p>
 </div>
 <div class="paragraph">
-<p>Some arithmetic operators can also be used with strings.</p>
+<p>Some arithmetic operators also apply to strings.</p>
 </div>
 <table class="tableblock frame-all grid-all stretch">
 <caption class="title">Table 3. String operators</caption>
@@ -1393,6 +1393,12 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 <td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">2 in [1,2,3]</code> &#8594; <em>true</em><br>
 <code class="blue">6 in [1,2,3]</code> &#8594; <em>false</em></p></td>
 </tr>
+<tr>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">#</code></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Size</em></p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock">Number of items in a list</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">#[1,2,3]</code> &#8594; <em>3</em></p></td>
+</tr>
 </tbody>
 </table>
 <div class="paragraph">
@@ -1415,11 +1421,19 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 </div>
 </div>
 <div class="paragraph">
-<div class="title">Items of list</div>
+<div class="title">Examples: Getting items from lists</div>
 <p><code>&gt;&gt;&gt;</code> <code class="blue">[1,2,3][1]</code><br>
 <code class="green">2</code></p>
 </div>
 <div class="paragraph">
+<p><code>&gt;&gt;&gt;</code> <code class="blue">index=2; ["a", "b", "c", "d"][index]</code><br>
+<code class="green">c</code></p>
+</div>
+<div class="paragraph">
+<p><code>&gt;&gt;&gt;</code> <code class="blue">["a", "b", "c", "d"][2:]</code><br>
+<code class="green">["c", "d"]</code></p>
+</div>
+<div class="paragraph">
 <p><code>&gt;&gt;&gt;</code> <code class="blue">list=[1,2,3]; list[1]</code><br>
 <code class="green">2</code></p>
 </div>
@@ -1432,6 +1446,10 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 <code class="green">two</code></p>
 </div>
 <div class="paragraph">
+<p><code>&gt;&gt;&gt;</code> <code class="blue">list[1]="six"; list</code><br>
+<code class="green">["one", "six", "three"]</code></p>
+</div>
+<div class="paragraph">
 <p><code>&gt;&gt;&gt;</code> <code class="blue">list[-1]</code><br>
 <code class="green">three</code></p>
 </div>
@@ -1440,22 +1458,42 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 <code class="red">Eval Error: [1:9] index 10 out of bounds</code></p>
 </div>
 <div class="paragraph">
+<div class="title">Example: Number of elements in a list</div>
 <p><code>&gt;&gt;&gt;</code> <code class="blue">#list</code><br>
 <code class="green">3</code></p>
 </div>
 <div class="paragraph">
-<p><code>&gt;&gt;&gt;</code> <code class="blue">index=2; ["a", "b", "c", "d"][index]</code><br>
-<code class="green">c</code></p>
+<div class="title">Examples: Element insertion</div>
+<p><code>&gt;&gt;&gt;</code> <code class="blue">"first" &gt;&gt; list</code><br>
+<code class="green">["first", "one", "six", "three"]</code></p>
 </div>
 <div class="paragraph">
-<p><code>&gt;&gt;&gt;</code> <code class="blue">["a", "b", "c", "d"][2:]</code><br>
-<code class="green">["c", "d"]</code></p>
+<p><code>&gt;&gt;&gt;</code> <code class="blue">list &lt;&lt; "last"</code><br>
+<code class="green">["first", "one", "six", "three", "last"]</code></p>
+</div>
+<div class="paragraph">
+<div class="title">Examples: Element in list</div>
+<p><code>&gt;&gt;&gt;</code> <code class="blue">"six" in list</code><br>
+<code class="green">true</code></p>
+</div>
+<div class="paragraph">
+<p><code>&gt;&gt;&gt;</code> <code class="blue">"ten" in list</code><br>
+<code class="green">false</code></p>
+</div>
+<div class="paragraph">
+<div class="title">Examples: Concatenation and filtering</div>
+<p><code>&gt;&gt;&gt;</code> <code class="blue">[1,2,3] + ["one", "two", "three"]</code><br>
+<code class="green">[1, 2, 3, "one", "two", "three"]</code></p>
+</div>
+<div class="paragraph">
+<p><code>&gt;&gt;&gt;</code> <code class="blue">[1,2,3,4] - [2,4]</code><br>
+<code class="green">[1, 3]</code></p>
 </div>
 </div>
 <div class="sect2">
 <h3 id="_dictionaries"><a class="anchor" href="#_dictionaries"></a><a class="link" href="#_dictionaries">2.5. Dictionaries</a></h3>
 <div class="paragraph">
-<p>The <em>dictionary</em>, or <em>dict</em>, data-type is set of pairs <em>key/value</em>. It is also known as <em>map</em> or <em>associative array</em>.</p>
+<p>The <em>dictionary</em>, or <em>dict</em>, data-type represents sets  of pairs <em>key/value</em>. It is also known as <em>map</em> or <em>associative array</em>.</p>
 </div>
 <div class="paragraph">
 <p>Dictionary literals are sequences of pairs separated by comma <code class="blue">,</code> enclosed between brace brackets.</p>
@@ -1514,6 +1552,12 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 <td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">"one" in {"one":1, "two":2}</code> &#8594; <em>true</em><br>
 <code class="blue">"six" in {"one":1, "two":2}</code> &#8594; <em>false</em></p></td>
 </tr>
+<tr>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><code class="blue">#</code></p></td>
+<td class="tableblock halign-center valign-top"><p class="tableblock"><em>Size</em></p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock">Number of items in a dict</p></td>
+<td class="tableblock halign-left valign-top"><p class="tableblock"><code class="blue">#{1:"a",2:"b",3:"c"}</code> &#8594; <em>3</em></p></td>
+</tr>
 </tbody>
 </table>
 <div class="paragraph">
@@ -1537,6 +1581,10 @@ dev-expr <span class="nt">--</span> Expressions calculator v1.10.0<span class="o
 <p><code>&gt;&gt;&gt;</code> <code class="blue">d={"one":1, "two":2}; d["six"]=6; d</code><br>
 <code class="green">{"two": 2, "one": 1, "six": 6}</code></p>
 </div>
+<div class="paragraph">
+<p><code>&gt;&gt;&gt;</code> <code class="blue">#d</code><br>
+<code class="green">3</code></p>
+</div>
 </div>
 </div>
 </div>
@@ -1582,7 +1630,7 @@ The assign operator <code class="blue">=</code> returns the value assigned to th
 <code class="green">3</code></p>
 </div>
 <div class="paragraph">
-<p><code>&gt;&gt;&gt;</code> <code class="blue">x = 5.2 * (9-3)</code>  <em class="gray">// The assigned value has the typical approximation error of the float data-type</em><br>
+<p><code>&gt;&gt;&gt;</code> <code class="blue">x = 5.2 * (9-3)</code>  <em class="gray">// The assigned value here has the typical approximation error of the float data-type</em><br>
 <code class="green">31.200000000000003</code></p>
 </div>
 <div class="paragraph">
@@ -1751,9 +1799,9 @@ Technically <code class="blue">;</code> is not treated as a real operator. It ac
 </div>
 </div>
 <div class="sect2">
-<h3 id="_variable_default_value_and"><a class="anchor" href="#_variable_default_value_and"></a><a class="link" href="#_variable_default_value_and">4.5. Variable default value <code class="blue">??</code> and <code class="blue">?=</code></a></h3>
+<h3 id="_variable_default_value_and"><a class="anchor" href="#_variable_default_value_and"></a><a class="link" href="#_variable_default_value_and">4.5. Variable default value <code class="blue">??</code>, <code class="blue">?=</code>, and <code class="blue">?!</code></a></h3>
 <div class="paragraph">
-<p>The left operand of these two operators must be a variable. The right operator can be any expression. They return the value of the variable if this is defined; otherwise they return the value of the right expression.</p>
+<p>The left operand of first two operators, <code class="blue">??</code> and <code class="blue">?=</code>, must be a variable. The right operator can be any expression. They return the value of the variable if this is defined; otherwise they return the value of the right expression.</p>
 </div>
 <div class="admonitionblock important">
 <table>
@@ -1774,6 +1822,21 @@ If the left variable is defined, the right expression is not evaluated at all.
 <p>The <code class="blue">?=</code> assigns the calculated value of the right expression to the left variable.</p>
 </div>
 <div class="paragraph">
+<p>The third one, <code class="blue">?!</code>, is the alternate operator. If the variable on the left size is not defined, it returns <em class="blue">nil</em>. Otherwise it returns the result of the expressione on the right side.</p>
+</div>
+<div class="admonitionblock important">
+<table>
+<tr>
+<td class="icon">
+<i class="fa icon-important" title="Important"></i>
+</td>
+<td class="content">
+If the left variable is NOT defined, the right expression is not evaluated at all.
+</td>
+</tr>
+</table>
+</div>
+<div class="paragraph">
 <div class="title">Examples</div>
 <p><code>&gt;&gt;&gt;</code> <code class="blue">var ?? (1+2)</code><br>
 <code class="green">3</code></p>
@@ -2204,7 +2267,7 @@ These operators have a high priority, in particular higher than the operator <co
 </div>
 <div id="footer">
 <div id="footer-text">
-Last updated 2024-09-12 06:56:30 +0200
+Last updated 2024-09-14 11:58:38 +0200
 </div>
 </div>
 </body>

parser.go

@@ -417,6 +417,7 @@ func (parser *parser) parseGeneral(scanner *scanner, allowForest bool, allowVarR
 		case SymEqual:
 			// if err = checkPrevSymbol(lastSym, SymIdentifier, tk); err == nil {
 			currentTerm, err = tree.addToken(tk)
+			firstToken = true
 			// }
 		case SymFuncDef:
 			var funcDefTerm *term
@@ -483,7 +484,7 @@ func (parser *parser) parseGeneral(scanner *scanner, allowForest bool, allowVarR
 // 	return
 // }
 
-func (parser *parser) expandOpAssign(scanner * scanner, tree *ast, tk *Token, allowVarRef bool) (t *term, err error) {
+func (parser *parser) expandOpAssign(scanner *scanner, tree *ast, tk *Token, allowVarRef bool) (t *term, err error) {
 	var opSym Symbol
 	var opString string