theorem Th20:
  p<>q & p in P & q in P & p in Q & q in Q implies P = Q
proof
  assume that
A1: p<>q and
A2: p in P & q in P and
A3: p in Q & q in Q;
  Line(p,q) = P by A1,A2,Th19;
  hence thesis by A1,A3,Th19;
end;
