reserve a,b,p,x,x9,x1,x19,x2,y,y9,y1,y19,y2,z,z9,z1,z2 for object,
   X,X9,Y,Y9,Z,Z9 for set;
reserve A,D,D9 for non empty set;
reserve f,g,h for Function;
reserve A,B for set;
reserve x,y,i,j,k for object;

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;
