override an element in a list in prolog

Question

I have the following problem:

I have a list with 9 positions and I have this function putPiece(Piece, Board, Line, Column) that puts a Piece in a Position Line, Column of a Board (the list of 9 positions) I'm trying to check if any position is already filled so that I don't try to put anything in it since it isn't possible (right?) can you please help me make a function checkIfFree(Board, Line, Column) that check is the position (Line, Column) of the board is already occupied or not...

Answer 1

假设一个自由位置由一个自由变量var(X)应该可以解决该问题。