Ops... solved. The answer is ''Replace Design''. 1. Create new tool (type of Forms, of cource). 2. Create any stub form and view in this tool. 3. Choose File / Replace Design and import design from the tool with the described trouble. 4. That's all. Increment is resetted in the new tool. You can delete old tool and rename new tool.

One of the field in the custom form was created with the wrong data type. So, I try to delete it and recreate with the same name but Groove add increment (MyField0, MyField1, MyField2 etc) each time I create feild. That's OK I know because it's possible to change label anyway. The point is that I don't like to use identifiers with incremental ...