PostGIS polygon inversion function — allows to invert polygon with 900913 SRID

st_invert.sql

create function st_invert(geometry) returns setof geometry as
'SELECT st_difference(st_geomfromtext(''POLYGON((-20037508.3427892 147730758.194568,
                                                          20037508.3427892 147730758.194568,
                                                          20037508.3427892 -147730762.669922,
                                                          -20037508.3427892 -147730762.669922,
                                                          -20037508.3427892 147730758.194568))'', 900913), $1)'

language sql;