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 Th4:
  |.0*n.| = 0
proof
  thus |.0*n .| = sqrt Sum (n |-> 0^2) by RVSUM_1:56
    .= sqrt (n*0) by RVSUM_1:80
    .= 0;
end;
