theorem Th18:
  not p on K implies for x st x on K holds IncProj(K,p,K).x=x
proof
  assume
A1: not p on K;
  let x;
A2: ex X st p on X & x on X by INCPROJ:def 5;
  assume x on K;
  hence thesis by A1,A2,Def1;
end;
