aggregate bounded wstddev( float x, float w ) returns float {
   // 1st argument is the value, 2nd is the weight.
   float s0;
   float s1;
   float s2;
   action add( float x, float w ) {
      if (w != 0.0) {
         s0 := s0 + w;
         s1 := s1 + w*x;
         s2 := s2 + w*x*x;
      }
   }
   action remove( float x, float w ) {
      if (w != 0.0) {
         s0 := s0 - w;
         s1 := s1 - w*x;
         s2 := s2 - w*x*x;
      }
   }
   action value() returns float {
      if (s0 != 0.0) { return ((s2 - s1*s1/s0)/s0).sqrt(); }
      else { return float.NAN; }
   }
}