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