new operator "?!" (alternate value)

common-errors.go

@@ -53,3 +53,9 @@ func ErrInvalidParameterValue(funcName, paramName string, paramValue any) error
 func ErrWrongParamType(funcName, paramName, paramType string, paramValue any) error {
 	return fmt.Errorf("%s(): the %q parameter must be a %s, got a %s (%v)", funcName, paramName, paramType, TypeName(paramValue), paramValue)
 }
+
+// --- Operator errors
+
+func ErrLeftOperandMustBeVariable(leftTerm, opTerm *term) error {
+	return leftTerm.Errorf("left operand of %q must be a variable", opTerm.source())
+}