[idvusers] ISL spatial subsetting

IDV list,

I'm in the process of writing some ISL files, and I'm fairly new to this so 
please forgive me.  In creating a data source with the <datasource> tag, is 
there an attribute to subset the spatial domain?  I see the 'times' attribute, 
but it would be nice to be able to subset the spatial domain, too.  I guess I 
could use jython method to do the loading of a datasource with spatial 
subsetting, but I like the ease of ISL language and was wondering if spatial 
subsetting was an option in ISL or if I'm just stuck with jython to do this?  
Thanks!

Mike U