In the current symbol (19991011) library there are symbols named vdd-1.sym, vcc-1.sym, vee-1.sym, etc... which do not have a net= attribute inside, so you must attach the net= attribute yourself (in the schematic).
There also symbols named 5V-minus-1.sym, 12V-plus-1.sym, 9V-plus-1.sym etc... which have the appropriate net= attribute in them already (can be overridden though). You can use these symbol as examples of how to use the net= attribute.
Ales Hvezda 2006-01-23