theorem
  for u,v,w being Element of AS holds ex u1 being Element of AS st u<>u1
  & (v,w '//' u,u1 or v,w '//' u1,u)
proof
  let u,v,w;
  consider u1 such that
A1: u<>u1 and
A2: u,u1 '//' v,w by Def1;
  v,w '//' u,u1 or v,w '//' u1,u by A2,Def1;
  hence thesis by A1;
end;
