reserve s for set,
  i,j for natural Number,
  k for Nat,
  x,x1,x2,x3 for Real,
  r,r1,r2,r3,r4 for Real,
  F,F1,F2,F3 for real-valued FinSequence,
  R,R1,R2 for Element of i-tuples_on REAL;

theorem Th86:
  0 <= Sum sqr F
proof
  now
    let i be Nat such that i in dom sqr F;
    0 <= (F.i)^2 by XREAL_1:63;
    hence 0 <= (sqr F).i by VALUED_1:11;
  end;
  hence thesis by Th84;
end;
