Arguments are the N top-most elements of the stack. Top-most argument is first. Thus elements have to be pushed on stack in reverse order.
Operation will pop N elements from the stack.
|  NaryInstr (Oper* op)
 | NaryInstr | 
Caller must not delete / modify operation.
Parameters:
| op | operation of this instruction | 
|  ~NaryInstr ()
 | ~NaryInstr | 
|  INSTR_VISITABLE ()
 | INSTR_VISITABLE | 
| const Oper&  op ()
 | op | 
[const]
Returns: operation associated with this instruction.