theorem Th86:
  X c= Y & X misses Z implies X c= Y \ Z
proof
  assume
A1: X c= Y & X /\ Z = {};
  let x be object;
  assume x in X;
  then x in Y & not x in Z by A1,XBOOLE_0:def 4;
  hence thesis by XBOOLE_0:def 5;
end;
