theorem Th89:
  for f be Function, a,b,n,m be object holds
    (f +* (a .--> b) +* (m .--> n)).m = n
proof
  let f be Function, a,b,n,m be object;
  set mn=m .--> n;
  m in dom mn by TARSKI:def 1;
  hence (f +* (a .--> b) +* mn).m=mn.m by Th13
    .=n by FUNCOP_1:72;
end;
