several changes:

1. moved scan/symbol.go to new package sym and adapted all files that reference Symbol type and its values.
2. new type interval defined by begin, end and step values.
3. replaced operator-range.go with operator-interval.go.
4. replaced range operator begin:end with begin..end..step.
5. new implementation of sub-collection extraction based on new interval literal.

operand-selector-case.go

@@ -10,6 +10,7 @@ import (
 
 	"git.portale-stac.it/go-pkg/expr/kern"
 	"git.portale-stac.it/go-pkg/expr/scan"
+	"git.portale-stac.it/go-pkg/expr/sym"
 )
 
 // -------- selector case term
@@ -32,7 +33,7 @@ func (sc *selectorCase) String() string {
 }
 
 func newSelectorCaseTerm(row, col int, filterList *scan.Term, caseExpr kern.Expr) *scan.Term {
-	tk := scan.NewValueToken(row, col, scan.SymSelectorCase, "", &selectorCase{filterList: filterList, caseExpr: caseExpr})
+	tk := scan.NewValueToken(row, col, sym.SymSelectorCase, "", &selectorCase{filterList: filterList, caseExpr: caseExpr})
 	return &scan.Term{
 		Tk:       *tk,
 		Parent:   nil,