The rule priority is based on the order you define them: if 
possible rules will override any rules preceding them.

So in the following rules, you are actually not including 
"/dir/foo.heml" in the project, you are overriding it with 
"-/dir/*" and therefore excluding it from the project:

+/dir/foo.heml
-/dir/*


If you say:

-/dir/*
+/dir/foo.heml

then you are excluding anything from the /dir/ folder but 
the explicitly including /dir/foo.heml

Actually, I don't even think starting your rule with "/" 
will match correctly, because the URLs do not start with / 
they start with www.site.com.  So more accurately would be:

+www.site.com/dir/foo.heml
-www.site.com/dir/*

or

+*/dir/foo.heml
-*/dir/*