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 Th2:
  x - x = 0*n & x + -x = 0*n
proof
  thus x - x = 0*n by RVSUM_1:37;
  hence thesis;
end;
