User blog comment:B1mb0w/The R Function/@comment-25337554-20180305085245/@comment-10262436-20180305111727

r(0) can only be substituted in the case R(r(0),n) = R(n,n) or R(R(r(0),r(0)),n) = R(R(n,r(0)),n) therefore

R(R(1,r(0)),n) <> R(R(1,n),n)

R(R(1,r(0)),n) = R(R^r(0)(0,r(0)_*),n) = R(R^n(0,r(0)_*),n) = R(R^n(r(0)),n) = R(r(0) + n,n)