text: CREATE OR REPLACE FUNCTION BBox(x integer, y integer, zoom integer)
      RETURNS geometry AS
  $BODY$
  DECLARE
      max numeric := 6378137 * pi();
      res numeric := max * 2 / 2^zoom;
      bbox geometry;
  BEGIN
      return ST_MakeEnvelope(
          -max + (x * res),
          max - (y * res),
          -max + (x * res) + res,
          max - (y * res) - res,
          3857);
  END;
  $BODY$
    LANGUAGE plpgsql IMMUTABLE;
  