21 (1) Every one is a party to an offence who

(a) actually commits it;

(b) does or omits to do anything for the purpose of aiding any person to commit it; or

(c) abets any person in committing it.

(2) Where two or more persons form an intention in common to carry out an unlawful purpose and to assist each other therein and any one of them, in carrying out the common purpose, commits an offence, each of them who knew or ought to have known that the commission of the offence would be a probable consequence of carrying out the common purpose is a party to that offence.