iterator.go: new error function errInvalidDataSource()

iterator.go

@@ -4,7 +4,10 @@
 // iterator.go
 package expr
 
-import "fmt"
+import (
+	"errors"
+	"fmt"
+)
 
 type Iterator interface {
 	Next() (item any, err error) // must return io.EOF after the last item
@@ -17,3 +20,7 @@ type Iterator interface {
 func errNoOperation(name string) error {
 	return fmt.Errorf("no %q function defined in the data-source", name)
 }
+
+func errInvalidDataSource() error {
+	return errors.New("invalid data-source")
+}