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
 for x1,x2,x3,x4 be Point of V st x1,x2,x3,x4 are_collinear
 holds x1,x2,x3 are_collinear & x1,x2,x4 are_collinear;
