solved a number of problems highlighted by the syntax analyzer

iterator.go

@@ -5,7 +5,7 @@
 package expr
 
 import (
-	"errors"
+	// "errors"
 	"fmt"
 )
 
@@ -43,6 +43,6 @@ func errNoOperation(name string) error {
 	return fmt.Errorf("no %s() function defined in the data-source", name)
 }
 
-func errInvalidDataSource() error {
-	return errors.New("invalid data-source")
-}
+// func errInvalidDataSource() error {
+// 	return errors.New("invalid data-source")
+// }