reserve r,s,t,u for Real;
reserve V for RealLinearSpace,
  v,w for Point of V;
reserve x1,x2,x3,x4,y1,y2 for Element of V;

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;
