お題
非負整数m, nに対し、
f(m, n)= n+1 if m==0
f(m, n) = f(m-1, 1) if n == 0
f(m, n) = f(m-1, f(m, n-1)) otherwise
なるf(m, n)の値を求めよ

f(2, 1) = ?
f(4, 1) = ?