reserve i,j,k,n for Nat;
reserve x,x1,x2,x3,y1,y2,y3 for set;

theorem
  for A,a,b being set holds (A --> a) +* (A --> b) = A --> b
proof
  let A,a,b be set;
  set g = A --> b;
  dom (A --> a) c= dom g;
  hence thesis by FUNCT_4:19;
end;
