% Base case: if PlusExpr is atomic, then LeftPlusExpr is also PlusExpr
left_plus(PlusExpr, LeftPlusExpr) :-
    atomic(PlusExpr),
    LeftPlusExpr = PlusExpr.

% Recursive case: PlusExpr is in the form A+B, where A and B are unrestricted plus expressions
left_plus(A + B, LeftPlusExpr) :-
    % Recurse on A
    left_plus(A, LeftA),

    % If B is atomic, then LeftPlusExpr is LeftA + B
    (   atomic(B)
    ->  LeftPlusExpr = LeftA + B
    ;   % Else, B is in the form B1 + B2
        B = B1 + B2,

        % Recurse on B1 + B2
        left_plus(B1 + B2, LeftB),

        % Extract the left-most atomic from LeftB
        LeftB = LeftB1 + LeftB2,

        % LeftPlusExpr is LeftA + LeftB1 + LeftB2
        LeftPlusExpr = LeftA + LeftB1 + LeftB2
    ).