theorem Th139:
  ((x,y,w,z) --> (a,b,c,d)).z=d
proof
  set f=(x,y) --> (a,b),g=(w,z) --> (c,d);
A1: g.z=d by Th63;
A2: dom g = {w,z} by Th62;
  z in dom g by A2,TARSKI:def 2;
  hence thesis by A1,Th13;
end;
