reserve k,j,n for Nat,
  r for Real;
reserve x,x1,x2,y for Element of REAL n;
reserve f for real-valued FinSequence;
reserve p,p1,p2,p3 for Point of TOP-REAL n,
  x,x1,x2,y,y1,y2 for Real;
reserve p,p1,p2 for Point of TOP-REAL 2;

theorem
  -|[x1,y1]| = |[ -x1, -y1]|
proof
  |[x1,y1]|`1 = x1 & |[x1,y1]|`2 = y1 by FINSEQ_1:44;
  hence thesis by Th31;
end;
