:- dynamic player/2.

register(Move, _) :-
    sent_by(Email, Move),
    player(Email, Name),
    reply_to(Move, 'Already a player, named ~a', [Name]).

register(_, Name) :-
    player(Email, Name),
    reply_to(Move, 'Name taken by ~a', [Email]).

register(Move, Name) :-
    sent_by(Email, Move),
    assert(player(Email, Name)),
    reply_to(Move, 'Welcome to the game!').