Iterators now support generic operation interface based on methods HasOperation() and CallOperation().

Also fixed mul() function that called doAdd() instead of doMul().

iterator.go

@@ -4,8 +4,16 @@
 // iterator.go
 package expr
 
+import "fmt"
+
 type Iterator interface {
 	Next() (item any, err error) // must return io.EOF after the last item
 	Current() (item any, err error)
 	Index() int
+	HasOperation(name string) bool
+	CallOperation(name string, args []any) (value any, err error)
+}
+
+func errNoOperation(name string) error {
+	return fmt.Errorf("no %q function defined in the data-source", name)
 }