NNI ==> NonNegativeInteger

A:(NNI,NNI) -> NNI

A(m,n) ==
  m=0 => n+1
  m>0 and n=0 => A(m-1,1)
  m>0 and n>0 => A(m-1,A(m,n-1))

-- Example
matrix [[A(i,j) for i in 0..3] for j in 0..3]