theorem Th80:
 for L being line of V, x1,x2 st x1 <> x2 & x1 in L & x2 in L
  holds L = Line(x1,x2)
proof let L be line of V;
  ex x3,x4 st L = Line(x3,x4) by Def15;
 hence thesis by Th75;
end;
