insert_payload
Description:
Insert the payload at index idx in this.
 If idx is -1, the payload will be appended to this.
Parameters:
| this | |
| idx | an index | 
| payload | 
Returns:
| true on success | 
Insert the payload at index idx in this.
 If idx is -1, the payload will be appended to this.
| this | |
| idx | an index | 
| payload | 
| true on success |