let A = fn(m, n) {
    if m == 0 {n + 1}
    else if m > 0 and n == 0 {A(m - 1, 1)}
    else {A(m - 1, A(m, n - 1))}
}

print(A(0, 0))
print(A(3, 4))
print(A(3, 1))