Left Fold 2

Implement the function

val fold_left2 : ('acc -> 'a -> 'b -> 'acc) -> 'acc -> 'a list -> 'b list -> 'acc

so that fold_left2 op base l r is the result of left-associatively folding op over the lists l and r simultaneously, e.g., fold_left2 op base [a1;a2;a3;a4] [b1;b2;b3;b4] is equivalent to op (op (op (op base a1 b1) a2 b2) a3 b3) a4 b4. The behavior of the function is undefined if l and r are not the same length. Your solution must be tail recursive.