left_plus(Expr, Left) :- left_plus(Expr, 0, Left).

% Base case: return the left plus expression when the expression is an atomic
left_plus(Atomic, Acc, Acc + Atomic) :- atomic(Atomic).

% Recursive case: recurse left-to-right, accumulating atomics into a left plus expression accumulator
left_plus(Left + Right, Acc, Result) :-
    atomic(Left),
    left_plus(Right, Acc + Left, Result).

left_plus(Left + Right, Acc, Result) :-
    \+ atomic(Left),
    left_plus(Left, Acc, Acc2),
    left_plus(Right, Acc2, Result).