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 Th17:
  (x1+x2+x3)+(y1+y2+y3)=(x1+y1)+(x2+y2)+(x3+y3)
proof
  thus (x1+x2+x3)+(y1+y2+y3) = ((x1+x2)+(y1+y2))+(x3+y3) by Th16
    .= (x1+y1)+(x2+y2)+(x3+y3) by Th16;
end;
