theorem Th25:
  proj1 X \ proj1 Y c= proj1(X \ Y)
proof
 let x be object;
 assume
A1: x in proj1 X \ proj1 Y;
  then x in proj1 X by XBOOLE_0:def 5;
  then consider y such that
A2: [x,y] in X by Def12;
  not x in proj1 Y by A1,XBOOLE_0:def 5;
  then not [x,y] in Y by Def12;
  then [x,y] in X \ Y by A2,XBOOLE_0:def 5;
 hence thesis by Def12;
end;
