error:singleton variable[x] in pro-log.how to solve it?

Question

it is missionary -cannibal program and found error at line legal(3,x).legal(0,x).

%        : mandc(state(3,3,left), [state(3,3,left)], Path)?
mandc(state(0, 0, right), _, []).
mandc(CurrentState, Visited, [Move | RestOfMoves]) :-
newstate(CurrentState, NextState),
not(member(NextState, Visited)),
make_move(CurrentState, NextState, Move),
mandc(NextState, [NextState | Visited], RestOfMoves).
% make_move(State1, State2, Move) builds the move(-,-,-) that gets you
% from State1 to State2.
make_move(state(M1, C1, left), state(M2, C2, right), move(M, C, right)) :-
        M is M1 - M2,
        C is C1 - C2.
make_move(state(M1, C1, right), state(M2, C2, left), move(M, C, left)) :-        M is M2 - M1,
        C is C2 - C1.
% carry(X,Y) is true if the boat can carry X missionaries and Y cannibals.
carry(2, 0).
carry(1, 0).
carry(1, 1).
carry(0, 1).
carry(0, 2).
% legal(X, Y) is true if it is safe for the missionaries to have X
% missionaries and Y cannibals together on a river bank (left or right).
legal(X, X).
**legal(3,X).
legal(0,X).**
% newstate(State1, State2)
newstate(state(M1, C1, left), state(M2, C2, right)) :-
carry(M, C),
        M =< M1,
        C =< C1,
        M2 is M1 - M,
        C2 is C1 - C,
legal(M2, C2).
newstate(state(M1, C1, right), state(M2, C2, left)) :-
carry(M, C),
        M2 is M1 + M,
        C2 is C1 + C,
        M2 =< 3,
        C2 =< 3,
legal(M2, C2).

can any one say solve it?if u have other program of missionary -cannibal then give me.

Answer 1

The problem is the two facts in question each have only one appearance of the variable X (hence, they are "singletons"). If you don't need to express that two (or more) things need to match, but just that it can be anything, use an underscore. Although this shouldn't be an error, but rather a warning.