The list operator '<<' (append) and '>>' (prepend) have been replaced

with the new operators '<+' and '+>' respectively.
'<<' and '>>' are now used only for left and right binary shit
respectively. Further, their priority has been increased moving them to
a higher priority than that of the assignment operator.

operator-shift.go

@@ -0,0 +1,85 @@
+// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// operator-shift.go
+package expr
+
+//-------- shift term
+
+func newRightShiftTerm(tk *Token) (inst *term) {
+	return &term{
+		tk:       *tk,
+		children: make([]*term, 0, 2),
+		position: posInfix,
+		priority: priBinShift,
+		evalFunc: evalRightShift,
+	}
+}
+
+func evalRightShift(ctx ExprContext, opTerm *term) (v any, err error) {
+	var leftValue, rightValue any
+
+	if leftValue, rightValue, err = opTerm.evalInfix(ctx); err != nil {
+		return
+	}
+
+	if IsInteger(leftValue) && IsInteger(rightValue) {
+		leftInt := leftValue.(int64)
+		rightInt := rightValue.(int64)
+		v = leftInt >> rightInt
+	} else {
+		err = opTerm.errIncompatibleTypes(leftValue, rightValue)
+	}
+	return
+}
+
+func newLeftShiftTerm(tk *Token) (inst *term) {
+	return &term{
+		tk:       *tk,
+		children: make([]*term, 0, 2),
+		position: posInfix,
+		priority: priBinShift,
+		evalFunc: evalLeftShift,
+	}
+}
+
+func evalLeftShift(ctx ExprContext, opTerm *term) (v any, err error) {
+	var leftValue, rightValue any
+
+	if leftValue, rightValue, err = opTerm.evalInfix(ctx); err != nil {
+		return
+	}
+
+	if IsInteger(leftValue) && IsInteger(rightValue) {
+		leftInt := leftValue.(int64)
+		rightInt := rightValue.(int64)
+		v = leftInt << rightInt
+	} else {
+		err = opTerm.errIncompatibleTypes(leftValue, rightValue)
+	}
+	return
+}
+
+// func evalAssignAppend(ctx ExprContext, self *term) (v any, err error) {
+// 	var leftValue, rightValue any
+
+// 	if leftValue, rightValue, err = self.evalInfix(ctx); err != nil {
+// 		return
+// 	}
+
+// 	if IsList(leftValue) {
+// 		list, _ := leftValue.(*ListType)
+// 		newList := append(*list, rightValue)
+// 		v = &newList
+// 		if
+// 	} else {
+// 		err = self.errIncompatibleTypes(leftValue, rightValue)
+// 	}
+// 	return
+// }
+
+// init
+func init() {
+	registerTermConstructor(SymDoubleGreater, newRightShiftTerm)
+	registerTermConstructor(SymDoubleLess, newLeftShiftTerm)
+}