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;

theorem Th6:
  |.f.| >= 0
proof
  0 <= Sum sqr f by RVSUM_1:86;
  hence thesis by SQUARE_1:def 2;
end;
