Accessing code within an external module in the same assembly

All this is just for practice. I am going to make a multi-modular
assembly comprising two modules as given below:

(a) a.vb compiled to a.netmodule (vbc a.vb /t:module)
(b) b.vb yet to be compiled to include the first module a.netmodule and
constitute an executable type assembly. (vbc b.vb /t:exe
/addmodule:a.netmodule /out:ab.exe)

If a.vb as a method called DoSomething and I want code inside b.vb to
access the DoSomething() that is inside a.vb, how do I do it? Is the
/r: (reference compiler switch available to modules also or only to
assemblies?