theorem Th33:
  proj1_3 X \ proj1_3 Y c= proj1_3(X \ Y)
proof
 let x be object;
 assume
A1: x in proj1_3 X \ proj1_3 Y;
  then x in proj1_3 X by XBOOLE_0:def 5;
  then consider y,z such that
A2: [x,y,z] in X by Th12;
  not x in proj1_3 Y by A1,XBOOLE_0:def 5;
  then not [x,y,z] in Y by Th13;
  then [x,y,z] in X \ Y by A2,XBOOLE_0:def 5;
 hence thesis by Th13;
end;
