moved a subset of source file to the kern package

operand-selector-case.go

@@ -7,6 +7,8 @@ package expr
 import (
 	"fmt"
 	"strings"
+
+	"git.portale-stac.it/go-pkg/expr/kern"
 )
 
 // -------- selector case term
@@ -41,7 +43,7 @@ func newSelectorCaseTerm(row, col int, filterList *term, caseExpr Expr) *term {
 }
 
 // -------- eval selector case
-func evalSelectorCase(ctx ExprContext, opTerm *term) (v any, err error) {
+func evalSelectorCase(ctx kern.ExprContext, opTerm *term) (v any, err error) {
 	var ok bool
 	if v, ok = opTerm.value().(*selectorCase); !ok {
 		err = fmt.Errorf("selector-case expected, got %T", opTerm.value())