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;

theorem Th4:
  x1 - (x2 - x3) = x1 - x2 + x3
proof
  thus x1 - (x2 - x3) = x1 - x2 - -x3 by RVSUM_1:39
    .= x1 - x2 + x3;
end;
