[postgis-users] st_area yields negative value for multi-part polygon

[george young]

PostgreSQL 9.1.4 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
4.4.4 20100630 (Red Hat 4.4.4-10), 64-bit
postgis-2.0.1, libgeos_c.so.1.7.5

gis=# SELECT st_area(ST_GeomFromText('POLYGON((-1 -1, 2 2, 0 1, -1
-1),(3 3, 5 5, 3 5, 3 3))'));
 st_area
---------
    -0.5

what does this mean?  What is negative area?
In general, what are the semantics of polygons with more than one
sequence of vertexes?