theorem Th19:
  not p on K & not p on L & x on K implies IncProj(K,p,L).x is POINT of IPP
proof
  assume ( not p on K)& not p on L & x on K;
  then x in dom IncProj(K,p,L) by Def1;
  hence thesis by PARTFUN1:4;
end;
