reserve x,A,B,X,X9,Y,Y9,Z,V for set;

theorem Th68:
  for A being non empty set st A c= Y & A c= Z holds Y meets Z
proof
  let A be non empty set;
  consider x being object such that
A1: x in A by XBOOLE_0:def 1;
  assume A c= Y & A c= Z;
  then x in Y & x in Z by A1;
  hence thesis by XBOOLE_0:3;
end;
