Returns the object that has sent the message under processing. This private method is translated into a unification between its argument and the corresponding implicit context argument in the predicate clause making the call. This unification occurs at the clause head when the argument is not instantiated (the most common case).
Modes and number of proofs
sender(?object_identifier) - zero_or_one
% after compilation, the write/1 call will % be the first goal on the clause body test :- sender(Sender), write('executing a method to answer a message sent by '), writeq(Sender), nl.