reserve r,s,t,u for Real;
reserve V for RealLinearSpace,
  v,w for Point of V;

theorem
 LSeg(v,w) c= Line(v,w)
proof let e be object;
 assume e in LSeg(v,w);
  then ex r st e =(1-r)*v + r*w & 0 <= r & r <= 1;
 hence e in Line(v,w);
end;
