Doc: little changes

doc/Expr.adoc

@@ -9,6 +9,7 @@ Expressions calculator
 :icons: font
 :icon-set: fi
 :numbered:
+:data-uri:
 //:table-caption: Tabella
 //:figure-caption: Diagramma
 :docinfo1:
@@ -19,10 +20,12 @@ Expressions calculator
 :rouge-style: gruvbox
 // :rouge-style: colorful
 //:rouge-style: monokay
+// Work around to manage double-column in back-tick quotes
+:2c: ::
 
 toc::[]
 
-#TODO: Work in progress (last update on 2024/06/17, 16:31 a.m.)#
+#TODO: Work in progress (last update on 2024/06/21, 05:40 a.m.)#
 
 == Expr
 _Expr_ is a GO package capable of analysing, interpreting and calculating expressions.
@@ -60,6 +63,8 @@ _Expr_ creates and keeps a inner _global context_ where it stores imported funct
 Imported functions are registerd in the _global context_. When an expression first calls an imported function, that function is linked to the current context; this can be the _main context_ or a _function context_.
 
 === `dev-expr` test tool
+Before we begin to describe the syntax of _Expr_, it is worth introducing _dev-expr_ because it will be used to show many examples of expressions.
+
 `dev-expr` is a simple program that can be used to evaluate expressions interactively. As its name suggests, it was created for testing purpose.  In fact, in additon to the automatic verification test suite based on the Go test framework, `dev-expr` provided an important aid for quickly testing of new features during their development.
 
 `dev-expr` can work as a _REPL_, _**R**ead-**E**xecute-**P**rint-**L**oop_, or it can process expression acquired from files or standard input.
@@ -101,7 +106,7 @@ dev-expr -- Expressions calculator v1.10.0(build 14),2024/06/17 (celestino.amoro
     -p                 Print prefix form
     -t                 Print tree form <2>
     -v, --version      Show program version
->>> 
+>>>
 ----
 
 <1> Only available for single fraction values
@@ -183,7 +188,7 @@ Value range: *-9223372036854775808* to *9223372036854775807*
 | [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
-|=== 
+|===
 
 ^(*)^ See also the _float division_ [blue]`./` below.
 
@@ -201,15 +206,20 @@ _dec-seq_ = _see-integer-literal-syntax_
 
 .Examples
 `>>>` [blue]`1.0` +
-[green]`1` +
+[green]`1`
+
 `>>>` [blue]`0.123` +
-[green]`0.123` +
+[green]`0.123`
+
 `>>>` [blue]`4.5e+3` +
-[green]`4500` +
+[green]`4500`
+
 `>>>` [blue]`4.5E-33` +
-[green]`4.5e-33` +
+[green]`4.5e-33`
+
 `>>>` [blue]`4.5E-3` +
-[green]`0.0045` +
+[green]`0.0045`
+
 `>>>` [blue]`4.5E10` +
 [green]`4.5e+10`
 
@@ -239,35 +249,43 @@ _digit-seq_ = _see-integer-literal-syntax_
 ====
 
 .Examples
-// [source,go]
-// ----
 `>>>` [blue]`1 | 2` +
-[green]`1|2` +
+[green]`1|2`
+
 `>>>` [blue]`4|6`  [gray]_// Fractions are always reduced to their lowest terms_ +
-[green]`2|3` +
+[green]`2|3`
+
 `>>>` [blue]`1|2 + 2|3` +
-[green]`7|6` +
+[green]`7|6`
+
 `>>>` [blue]`1|2 * 2|3` +
-[green]`1|3` +
+[green]`1|3`
+
 `>>>` [blue]`1|2 / 1|3` +
-[green]`3|2` +
+[green]`3|2`
+
 `>>>` [blue]`1|2 ./ 1|3` [gray]_// Force decimal division_ +
-[green]`1.5` +
+[green]`1.5`
+
 `>>>` [blue]`-1|2` +
-[green]`-1|2` +
+[green]`-1|2`
+
 `>>>` [blue]`1|-2` [gray]_// Invalid sign specification_ +
-[red]_Eval Error: [1:3] infix operator "|" requires two non-nil operands, got 1_ +
+[red]_Eval Error: [1:3] infix operator "|" requires two non-nil operands, got 1_
+
 `>>>` [blue]`1|(-2)` +
 [green]`-1|2`
-// ----
+
 
 Fractions can be used together with integers and floats in expressions.
 
 .Examples
 `>>>` [blue]`1|2 + 5` +
-[green]`11|2` +
+[green]`11|2`
+
 `>>>` [blue]`4 - 1|2` +
-[green]`7|2` +
+[green]`7|2`
+
 `>>>` [blue]`1.0 + 1|2` +
 [green]`1.5`
 
@@ -278,12 +296,15 @@ Strings are character sequences enclosed between two double quote [blue]`"`.
 
 .Examples
 `>>>` [blue]`"I'm a string"` +
-[green]`I'm a string` +
+[green]`I'm a string`
+
 `>>>` [blue]`"123abc?!"` +
-[green]`123abc?!` +
+[green]`123abc?!`
+
 `>>>` [blue]`"123\nabc"` +
 [green]`123` +
-[green]`abc` +
+[green]`abc`
+
 `>>>` [blue]`"123\tabc"` +
 [green]`123{nbsp}{nbsp}{nbsp}{nbsp}abc`
 
@@ -304,25 +325,30 @@ The items of strings can be accessed using the square `[]` operator.
 
 .Item access syntax
 ====
-_item_ = _string-expr_ "**[**" _integer-expr_ "**]**" 
+*_item_* = _string-expr_ "**[**" _integer-expr_ "**]**"
 ====
 
-.Sub string syntax
+.Sub-string syntax
 ====
-_sub-string_ = _string-expr_ "**[**" _integer-expr_ "**:**" _integer-expr_ "**]**" 
+*_sub-string_* = _string-expr_ "**[**" _integer-expr_ "**:**" _integer-expr_ "**]**"
 ====
 
 .String examples
 `>>>` [blue]`s="abcd"`  [gray]_// assign the string to variable s_ +
-[green]`"abcd"` +
+[green]`"abcd"`
+
 `>>>` [blue]`s[1]`      [gray]_// char at position 1 (starting from 0)_ +
-[green]`"b"` +
+[green]`"b"`
+
 `>>>` [blue]`s.[-1]`    [gray]_// char at position -1, the rightmost one_ +
-[green]`"d"` +
+[green]`"d"`
+
 `>>>` [blue]`\#s`       [gray]_// number of chars_ +
-[gren]`4` +
+[gren]`4`
+
 `>>>` [blue]`#"abc"`    [gray]_// number of chars_ +
-[green]`3` +
+[green]`3`
+
 `>>>` [blue]`s[1:3]`    [gray]_// chars from position 1 to position 3 excluded_ +
 [grean]`"bc"`
 
@@ -368,14 +394,16 @@ Boolean data type has two values only: [blue]_true_ and [blue]_false_. Relationa
 
 [CAUTION]
 ====
-Currently, boolean operations are evaluated using _short cut evaluation_. This means that, if the left expression of the [blue]`and` and [blue]`or` operators is sufficient to establish the result of the whole operation, the right expression would not evaluated at all.
-
+Currently, boolean operations are evaluated using _short cut evaluation_. This means that, if the left expression of the [blue]`and` and [blue]`or` operators is sufficient to establish the result of the whole operation, the right expression would not be evaluated at all.
 .Example
 [source,go]
 ----
 2 > (a=1) or (a=8) > 0; a // <1>
 ----
 <1> This multi-expression returns _1_ because in the first expression the left value of [blue]`or` is _true_ and as a conseguence its right value is not computed. Therefore the _a_ variable only receives the integer _1_.
+
+
+TIP: `dev-expr` provides the _ctrl()_ function that allows to change this behaviour.
 ====
 
 === Lists
@@ -390,13 +418,17 @@ _non-empty-list_ = "**[**" _any-value_ {"**,**" _any-value} "**]**" +
 
 .Examples
 `>>>` [blue]`[1,2,3]`                     [gray]_// List of integers_ +
-[green]`[1, 2, 3]` +
+[green]`[1, 2, 3]`
+
 `>>>` [blue]`["one", "two", "three"]`     [gray]_// List of strings_ +
-[green]`["one", "two", "three"]` +
+[green]`["one", "two", "three"]`
+
 `>>>` [blue]`["one", 2, false, 4.1]`      [gray]_// List of mixed-types_ +
-[green]`["one", 2, false, 4.1]` +
+[green]`["one", 2, false, 4.1]`
+
 `>>>` [blue]`["one"+1, 2.0*(9-2)]`        [gray]_// List of expressions_ +
-[green]`["one1", 14]` +
+[green]`["one1", 14]`
+
 `>>>` [blue]`[ [1,"one"], [2,"two"]]`     [gray]_// List of lists_ +
 [green]`[[1, "one"], [2, "two"]]`
 
@@ -409,37 +441,52 @@ _non-empty-list_ = "**[**" _any-value_ {"**,**" _any-value} "**]**" +
 | [blue]`-` | _Difference_ | Left list without elements in the right list  | [blue]`[1,2,3] - [2]` -> _[1,3]_
 | [blue]`>>` | _Front insertion_ | Insert an item in front | [blue]`0 >> [1,2]` -> _[0,1,2]_
 | [blue]`<<` | _Back insertion_ | Insert an item at end | [blue]`[1,2] << 3` -> _[1,2,3]_
-| [blue]`.` | _List item_ | Item at given position | [blue]`[1,2.3].1` -> _2_
+| [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_
 |===
 
-The items of array can be accessed using the dot `.` operator.
+Array's items can be accessed using the index `[]` operator.
 
 .Item access syntax
 ====
-_item_ = _list-expr_ "**.**" _integer-expr_
+*_item_* = _list-expr_ "**[**" _integer-expr_ "**]**"
+====
+
+.Sub-array (or slice of array) syntax
+====
+*_slice_* = _string-expr_ "**[**" _integer-expr_ "**:**" _integer-expr_ "**]**"
 ====
 
 .Items of list
 `>>>` [blue]`[1,2,3].1` +
-[green]`2` +
+[green]`2`
+
 `>>>` [blue]`list=[1,2,3]; list.1` +
-[green]`2` +
+[green]`2`
+
 `>>>` [blue]`["one","two","three"].1` +
-[green]`two` +
+[green]`two`
+
 `>>>` [blue]`list=["one","two","three"]; list.(2-1)` +
-[green]`two` +
+[green]`two`
+
 `>>>` [blue]`list.(-1)` +
-[green]`three` +
+[green]`three`
+
 `>>>` [blue]`list.(10)` +
-[red]`Eval Error: [1:9] index 10 out of bounds` +
+[red]`Eval Error: [1:9] index 10 out of bounds`
+
 `>>>` [blue]`#list` +
-[green]`3` +
-`>>>` [blue]`index=2; ["a", "b", "c", "d"].index` +
+[green]`3`
+
+`>>>` [blue]`index=2; ["a", "b", "c", "d"][index]` +
 [green]`c`
 
 
+`>>>` [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_.
@@ -453,14 +500,11 @@ _empty-dict_ = "**{}**" +
 _non-empty-dict_ = "**{**" _key-scalar_ "**:**" _any-value_ {"**,**" _key-scalar_ "**:**" _any-value} "**}**" +
 ====
 
-.Examples
-`>>>` [blue]`{1:"one", 2:"two"}` +
-[green]`{1: "one", 2: "two"}` +
-`>>>` [blue]`{"one":1, "two": 2}` +
-[green]`{"one": 1, "two": 2}` +
-`>>>` [blue]`{"sum":1+2+3, "prod":1*2*3}` +
-[green]`{"sum": 6, "prod": 6}`
 
+.Item access syntax
+====
+*_item_* = _dict-expr_ "**[**" _key-expr_ "**]**"
+====
 
 .Dict operators
 [cols="^2,^2,4,5"]
@@ -468,11 +512,27 @@ _non-empty-dict_ = "**{**" _key-scalar_ "**:**" _any-value_ {"**,**" _key-scalar
 | Symbol    | Operation     | Description               | Examples
 
 | [blue]`+` | _Join_ | Joins two dicts | [blue]`{1:"one"}+{6:"six"}` -> _{1: "one", 6: "six"}_
-| [blue]`.` | _Dict item value_ | Item value of given key | [blue]`{"one":1, "two":2}."two"` -> _2_
+| [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_
 |===
 
+.Examples
+`>>>` [blue]`{1:"one", 2:"two"}` +
+[green]`{1: "one", 2: "two"}`
+
+`>>>` [blue]`{"one":1, "two": 2}` +
+[green]`{"one": 1, "two": 2}`
+
+`>>>` [blue]`{"sum":1+2+3, "prod":1*2*3}` +
+[green]`{"sum": 6, "prod": 6}`
+
+`>>>` [blue]`{"one":1, "two":2}["two"]` +
+[green]`2`
+
+`>>>` [blue]`d={"one":1, "two":2}; d["six"]=6; d` +
+[green]`{"two": 2, "one": 1, "six": 6}`
+
 
 == Variables
 _Expr_, like most programming languages, supports variables. A variable is an identifier with an assigned value. Variables are stored in _contexts_.
@@ -488,17 +548,23 @@ NOTE: The assign operator [blue]`=` returns the value assigned to the variable.
 
 .Examples
 `>>>` [blue]`a=1` +
-[green]`1` +
+[green]`1`
+
 `>>>` [blue]`a_b=1+2` +
-[green]`1+2` +
+[green]`1+2`
+
 `>>>` [blue]`a_b` +
-[green]`3` +
-`>>>` [blue]`x = 5.2 * (9-3)`  [gray]_// The assigned value has the approximation error typical of the float data-type_ +
-[green]`31.200000000000003` +
+[green]`3`
+
+`>>>` [blue]`x = 5.2 * (9-3)`  [gray]_// The assigned value has the typical approximation error of the float data-type_ +
+[green]`31.200000000000003`
+
 `>>>` [blue]`x = 1; y = 2*x` +
-[green]`2` +
+[green]`2`
+
 `>>>` [blue]`_a=2` +
-[red]`Parse Error: [1:2] unexpected token "_"` +
+[red]`Parse Error: [1:2] unexpected token "_"`
+
 `>>>` [blue]`1=2` +
 [red]`Parse Error: assign operator ("=") must be preceded by a variable`
 
@@ -508,6 +574,11 @@ NOTE: The assign operator [blue]`=` returns the value assigned to the variable.
 === [blue]`;` operator
 The semicolon operator [blue]`;` is an infixed pseudo-operator. It evaluates the left expression first and then the right expression. The value of the latter is the final result.
 
+.Mult-expression syntax
+====
+*_multi-expression_* = _expression_ {"**;**" _expression_ }
+====
+
 An expression that contains [blue]`;` is called a _multi-expression_ and each component expressione is called a _sub-expression_.
 
 IMPORTANT: Technically [blue]`;` is not treated as a real operator. It acts as a separator in lists of expressions.
@@ -566,38 +637,47 @@ The [blue]`:` symbol (colon) is the separator of the selector-cases. Note that i
 
 .Examples
 `>>>` [blue]`1 ? {"a"} : {"b"}` +
-[green]`b` +
-`>>>` [blue]`10 ? {"a"} : {"b"} :: {"c"}` +
-[green]`c' +
-[green]`>>>` [blue]`10 ? {"a"} :[true, 2+8] {"b"} :: {"c"}` +
-[green]`b` +
-`>>>` [blue]`10 ? {"a"} :[true, 2+8] {"b"} ::[10] {"c"}` +
-[red]`Parse Error: [1:34] case list in default clause` +
-[green]`>>>` [blue]`10 ? {"a"} :[10] {x="b" but x} :: {"c"}` +
-[green]`b` +
-`>>>` [blue]`10 ? {"a"} :[10] {x="b"; x} :: {"c"}` +
-[green]`b` +
+[green]`b`
+
+`>>>` [blue]`10 ? {"a"} : {"b"} {2c} {"c"}` +
+[green]`c`
+
+`>>>` [blue]`10 ? {"a"} :[true, 2+8] {"b"} {2c} {"c"}` +
+[green]`b`
+
+`>>>` [blue]`10 ? {"a"} :[true, 2+8] {"b"} {2c} [10] {"c"}` +
+[red]`Parse Error: [1:34] case list in default clause`
+
+`>>>` [blue]`10 ? {"a"} :[10] {x="b" but x} {2c} {"c"}` +
+[green]`b`
+
+`>>>` [blue]`10 ? {"a"} :[10] {x="b"; x} {2c} {"c"}` +
+[green]`b`
+
 `>>>` [blue]`10 ? {"a"} : {"b"}` +
 [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 define; otherwise they return the value of the right expression.
+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.
 
-IMPORTANT: If the left variable is defined, the right expression is not evuated at all.
+IMPORTANT: If the left variable is defined, the right expression is not evaluated at all.
 
-The [blue]`??` do not change the status of the left variable.
+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.
 
 .Examples
-`>>>` [blue]`var ?? (1+2)`'
-[green]`3` +
+`>>>` [blue]`var ?? (1+2)`' +
+[green]`3`
+
 `>>>` [blue]`var` +
-[red]`Eval Error: undefined variable or function "var"` +
+[red]`Eval Error: undefined variable or function "var"`
+
 `>>>` [blue]`var ?= (1+2)` +
-[green]`3` +
-`>>>` [blue]`var` +
+[green]`3`
+
+`>>>` [blue]`var`
 [green]`3`
 
 NOTE: These operators have a high priority, in particular higher than the operator [blue]`=`.
@@ -610,57 +690,87 @@ The table below shows all supported operators by decreasing priorities.
 |===
 | Priority | Operators     | Position | Operation | Operands and results
 
-.2+|*ITEM*| [blue]`.` | _Infix_ | _List item_| _list_ `"."` _integer_ -> _any_
-| [blue]`.` | _Infix_ | _Dict item_ | _dict_ `"."` _any_ -> _any_
-.2+|*INC*| [blue]`++` | _Postfix_ | _Post increment_| _integer-variable_ `"++"` -> _integer_
-| [blue]`++` | _Postfix_ | _Next item_ | _iterator_ `"++"` -> _any_
-.1+|*FACT*| [blue]`!` | _Postfix_ | _Factorial_| _integer_ `"!"` -> _integer_
-.3+|*SIGN*| [blue]`+`, [blue]`-` | _Prefix_ | _Change-sign_| (`"+"`\|`"-"`) _number_ -> _number_
-| [blue]`#` | _Prefix_ | _Lenght-of_ | `"#"` _collection_ -> _integer_
-| [blue]`#` | _Prefix_ | _Size-of_ | `"#"` _iterator_ -> _integer_
-.5+|*PROD*| [blue]`*` | _Infix_ | _Product_ | _number_ `"*"` _number_ -> _number_
-| [blue]`*` | _Infix_ | _String-repeat_ | _string_ `"*"` _integer_ -> _string_
-| [blue]`/` | _Infix_ | _Division_ | _number_ `"/"` _number_ -> _number_
-| [blue]`./` | _Infix_ | _Float-division_ | __number__ `"./"` _number_ -> _float_
-| [blue]`%` | _Infix_ | _Integer-remainder_ | _integer_ `"%"` _integer_ -> _integer_
-.6+|*SUM*| [blue]`+` | _Infix_ | _Sum_ | _number_ `"+"` _number_ -> _number_
-| [blue]`+` | _Infix_ |  _String-concat_ | (_string_\|_number_) `"+"` (_string_\|_number_) -> _string_
-| [blue]`+` | _Infix_ |  _List-join_ | _list_ `"+"` _list_ -> _list_
-| [blue]`+` | _Infix_ |  _Dict-join_ | _dict_ `"+"` _dict_ -> _dict_
-| [blue]`-` | _Infix_ | _Subtraction_ | _number_ `"-"` _number_ -> _number_
-| [blue]`-` | _Infix_ | _List-difference_ | _list_ `"-"` _list_ -> _list_
-.8+|*RELATION*| [blue]`<` | _Infix_ | _less_ | _comparable_ `"<"` _comparable_ -> _boolean_
-| [blue]`\<=` | _Infix_ |  _less-equal_ | _comparable_ `"\<="` _comparable_ -> _boolean_
-| [blue]`>` | _Infix_ |  _greater_ | _comparable_ `">"` _comparable_ -> _boolean_
-| [blue]`>=` | _Infix_ |  _greater-equal_ | _comparable_ `">="` _comparable_ -> _boolean_
-| [blue]`==` | _Infix_ |  _equal_ | _comparable_ `"=="` _comparable_ -> _boolean_
-| [blue]`!=` | _Infix_ |  _not-equal_ | _comparable_ `"!="` _comparable_ -> _boolean_
-| [blue]`in` | _Infix_ |  _member-of-list_ | _any_ `"in"` _list_ -> _boolean_
-| [blue]`in` | _Infix_ |  _key-of-dict_ | _any_ `"in"` _dict_ -> _boolean_
-.1+|*NOT*| [blue]`not` | _Prefix_ | _not_ | `"not"` _boolean_ -> _boolean_
-.2+|*AND*| [blue]`and` | _Infix_ | _and_ | _boolean_ `"and"` _boolean_ -> _boolean_
-| [blue]`&&` | _Infix_ | _and_ | _boolean_ `"&&"` _boolean_ -> _boolean_
-.2+|*OR*| [blue]`or` | _Infix_ | _or_ | _boolean_ `"or"` _boolean_ -> _boolean_
-| [blue]`\|\|` | _Infix_ | _or_ | _boolean_ `"\|\|"` _boolean_ -> _boolean_
-.3+|*ASSIGN*| [blue]`=` | _Infix_ | _assignment_ | _identifier_ "=" _any_ -> _any_
-| [blue]`>>` | _Infix_ | _front-insert_ | _any_ ">>" _list_ -> _list_
-| [blue]`<<` | _Infix_ | _back-insert_ | _list_ "<<" _any_ -> _list_
- .1+|*BUT*| [blue]`but` | _Infix_ | _but_ | _any_ "but" _any_ -> _any_
+.2+|*ITEM*| [blue]`[`...`]` | _Postfix_ | _List item_| _list_ `[`  _integer_ `]` -> _any_
+| [blue]`[`...`]` | _Postfix_ | _Dict item_ | _dict_ `[` _any_ `]` -> _any_
+.2+|*INC*| [blue]`++` | _Postfix_ | _Post increment_| _integer-variable_ `++` -> _integer_
+| [blue]`++` | _Postfix_ | _Next item_ | _iterator_ `++` -> _any_
+.2+|*DEFAULT*| [blue]`??` | _Infix_ | _Default value_| _variable_ `??` _any-expr_ -> _any_
+| [blue]`?=` | _Infix_ | _Default/assign value_| _variable_ `?=` _any-expr_ -> _any_
+.1+| *ITER*^1^| [blue]`()` | _Prefix_ | _Iterator value_ | `()` _iterator_ -> _any_
+.1+|*FACT*| [blue]`!` | _Postfix_ | _Factorial_| _integer_ `!` -> _integer_
+.3+|*SIGN*| [blue]`+`, [blue]`-` | _Prefix_ | _Change-sign_| (`+`\|`-`) _number_ -> _number_
+| [blue]`#` | _Prefix_ | _Lenght-of_ | `#` _collection_ -> _integer_
+| [blue]`#` | _Prefix_ | _Size-of_ | `#` _iterator_ -> _integer_
+.2+|*SELECT*| [blue]`? : ::` | _Multi-Infix_ | _Case-Selector_ | _any-expr_ `?` _case-list_ _case-expr_ `:` _case-list_ _case-expr_ ... `::` _default-expr_ -> _any_
+| [blue]`? : ::` | _Multi-Infix_ | _Index-Selector_ | _int-expr_ `?` _case-expr_ `:` _case-expr_ ... `::` _default-expr_ -> _any_
+.1+|*FRACT*| [blue]`\|` | _Infix_ | _Fraction_ | _integer_ `\|` _integer_ -> _fraction_
+.5+|*PROD*| [blue]`*` | _Infix_ | _Product_ | _number_ `*` _number_ -> _number_
+| [blue]`*` | _Infix_ | _String-repeat_ | _string_ `*` _integer_ -> _string_
+| [blue]`/` | _Infix_ | _Division_ | _number_ `/` _number_ -> _number_
+| [blue]`./` | _Infix_ | _Float-division_ | __number__ `./` _number_ -> _float_
+| [blue]`%` | _Infix_ | _Integer-remainder_ | _integer_ `%` _integer_ -> _integer_
+.6+|*SUM*| [blue]`+` | _Infix_ | _Sum_ | _number_ `+` _number_ -> _number_
+| [blue]`+` | _Infix_ |  _String-concat_ | (_string_\|_number_) `+` (_string_\|_number_) -> _string_
+| [blue]`+` | _Infix_ |  _List-join_ | _list_ `+` _list_ -> _list_
+| [blue]`+` | _Infix_ |  _Dict-join_ | _dict_ `+` _dict_ -> _dict_
+| [blue]`-` | _Infix_ | _Subtraction_ | _number_ `-` _number_ -> _number_
+| [blue]`-` | _Infix_ | _List-difference_ | _list_ `-` _list_ -> _list_
+.8+|*RELATION*| [blue]`<` | _Infix_ | _Less_ | _comparable_ `<` _comparable_ -> _boolean_
+| [blue]`\<=` | _Infix_ |  _less-equal_ | _comparable_ `\<=` _comparable_ -> _boolean_
+| [blue]`>` | _Infix_ |  _Greater_ | _comparable_ `>` _comparable_ -> _boolean_
+| [blue]`>=` | _Infix_ |  _Greater-equal_ | _comparable_ `>=` _comparable_ -> _boolean_
+| [blue]`==` | _Infix_ |  _Equal_ | _comparable_ `==` _comparable_ -> _boolean_
+| [blue]`!=` | _Infix_ |  _Not-equal_ | _comparable_ `!=` _comparable_ -> _boolean_
+| [blue]`in` | _Infix_ |  _Member-of-list_ | _any_ `in` _list_ -> _boolean_
+| [blue]`in` | _Infix_ |  _Key-of-dict_ | _any_ `in` _dict_ -> _boolean_
+.1+|*NOT*| [blue]`not` | _Prefix_ | _Not_ | `not` _boolean_ -> _boolean_
+.2+|*AND*| [blue]`and` | _Infix_ | _And_ | _boolean_ `and` _boolean_ -> _boolean_
+| [blue]`&&` | _Infix_ | _And_ | _boolean_ `&&` _boolean_ -> _boolean_
+.2+|*OR*| [blue]`or` | _Infix_ | _Or_ | _boolean_ `or` _boolean_ -> _boolean_
+| [blue]`\|\|` | _Infix_ | _Or_ | _boolean_ `\|\|` _boolean_ -> _boolean_
+.3+|*ASSIGN*| [blue]`=` | _Infix_ | _Assignment_ | _identifier_ `=` _any_ -> _any_
+| [blue]`>>` | _Infix_ | _Front-insert_ | _any_ `>>` _list_ -> _list_
+| [blue]`<<` | _Infix_ | _Back-insert_ | _list_ `<<` _any_ -> _list_
+.1+|*BUT*| [blue]`but` | _Infix_ | _But_ | _any_ `but` _any_ -> _any_
+.1+|*RANGE*| [blue]`:` | _Infix_ | _Index-range_ | _integer_ `:` _integer_ -> _integer-pair_
 |===
 
+^1^ Experimental
+
+
 == Functions
 Functions in _Expr_ are very similar to functions available in many programming languages. Actually, _Expr_ supports two types of function, _expr-functions_ and _go-functions_.
 
 * _expr-functions_ are defined using _Expr_'s syntax. They can be passed as arguments to other functions and can be returned from functions. Moreover, they bind themselves to the defining context, thus becoming closures.
 * _go-functions_ are regular Golang functions callable from _Expr_ expressions. They are defined in Golang source files called _modules_ and compiled within the _Expr_ package. To make Golang functions available in _Expr_ contextes, it is required to _import_ the module in which they are defined.
 
-In _Expr_ functions compute values in a local context (scope) that do not make effects on the calling context. This is the normal behavior. Using the reference operator [blue]`@` it is possibile to export local definition to the calling context.
+
+=== _Expr_ function definition
+A function is identified and referenced by its name. It can have zero or more parameter. _Expr_ functions also support optional parameters.
+
+. Expr's function definition syntax
+====
+*_function-definition_* = _identifier_ "**=**"  "**func(**" [_param-list_] "**)**"  "**{**" _multi-expression_ "**}**"
+_param_list_ = _required-param-list_ [ "**,**" _optional-param-list_ ]
+_required-param-list_ = _identifier_ { "**,**" _identifier_ }
+_optional-param-list_ = _optional-parm_ { "**,**" _optional-param_ }
+_optional-param_ = _identifier_ "**=**" _any-expr_
+====
+
+.Examples
+#TODO#
+
+=== _Golang_ function definition
+Description of how to define Golan functions and how to bind them to _Expr_ are topics treated in another document that I'll write, one day, maybe.
 
 === Function calls
 #TODO: function calls operations#
 
-=== Function definitions
-#TODO: function definitions operations#
+Functions compute values in a local context (scope) that do not make effects on the calling context. This is the normal behavior. Using the reference operator [blue]`@` it is possibile to export local definition to the calling context.
+
+
+== Iterators
+#TODO: function calls operations#
 
 == Builtins
 #TODO: builtins#
@@ -671,4 +781,7 @@ In _Expr_ functions compute values in a local context (scope) that do not make e
 [blue]_import([grey]#<source-file>#)_ loads the multi-expression contained in the specified source and returns its value.
 
 
+== Plugins
+#TODO: plugins#
+