theorem Th12:
  <*x/"y*>^<*y*> ==>. x & <*y*>^<*y\x*> ==>. x
proof
A1: <*x*> ==>. x by Def18;
  <*y*> ==>. y by Def18;
  hence thesis by A1,Lm4,Lm7;
end;
