def record_modify_controlfield(rec, tag, controlfield_value,<EOL>field_position_global=None,<EOL>field_position_local=None):
field = record_get_field(<EOL>rec, tag,<EOL>field_position_global=field_position_global,<EOL>field_position_local=field_position_local)<EOL>new_field = (field[<NUM_LIT:0>], field[<NUM_LIT:1>], field[<NUM_LIT:2>], controlfield_value, field[<NUM_LIT:4>])<EOL>record_replace_field(<EOL>rec, tag, new_field,<EOL>field_position_global=field_position_global,<EOL>field_position_local=field_position_local)<EOL>
Modify controlfield at position specified by tag and field number.