reserve a,a1,a2,a3,b,b1,b2,b3,r,s,t,u for Real;
reserve n for Nat;
reserve x0,x,x1,x2,x3,y0,y,y1,y2,y3 for Element of REAL n;
reserve L,L0,L1,L2 for Element of line_of_REAL n;

theorem
  L = Line(x1,x2) implies {|.x-x0.| where x0 is Element of REAL n : x0
  in Line(x1,x2)} = dist_v(x,L);
