theorem Th20:
  not p on K & not p on L & x on K & y = IncProj (K,p,L).x implies y on L
proof
  assume
A1: ( not p on K)& not p on L & x on K & y = IncProj(K,p,L).x;
  consider X such that
A2: p on X & x on X by INCPROJ:def 5;
  ex z being POINT of IPP st z on L & z on X by INCPROJ:def 9;
  hence thesis by A1,A2,Def1;
end;
