each of the following recv_internal declarations is correct, but those with fewer variables will spend slightly less gas (each unused argument adds additional DROP instructions).

() recv_internal(int balance, int msg_value, cell in_msg_cell, slice in_msg) {}
() recv_internal(int msg_value, cell in_msg_cell, slice in_msg) {}
() recv_internal(cell in_msg_cell, slice in_msg) {}
() recv_internal(slice in_msg) {}

You have to parse in_msg_body to retrieve the op code based on how sender stored op code in the message.

Ususally, it is stored as the first 32 bit inside in_msg_body.