velocimacro.library - all specified required?

I’m trying to set up a system that has some built-in (to a JAR, class path loader) macros defined (in _macros.vm) and allows the user, in their own file system loader path, to define macros.vm (without underscore prefix) to add their own.

I’m trying this:
   engine.setProperty(RuntimeConstants.VM_LIBRARY, "_macros.vm,macros.vm”);

But I get errors if macros.vm is not found.

Can I make it so that velocimacro.library loaded files are optional, such that if macros.vm isn’t there all works fine?   It’s giving me an error currently. 

Also, when given that comma-separated list, are they loaded in that order such that a macro defined in macros.vm can override one in _macros.vm?

Thanks for your help!

	Erik