[CMake] Double checking DEPENDS and MAIN_DEPENDS behavior

[James Bigler]

I have a ADD_CUSTOM_COMMAND that has a series of dependencies.  I just want to 
make sure that I understand what I should use.

ADD_CUSTOM_COMMAND(
   OUTPUT ..
   COMMAND ..
   ARGS ..
   MAIN_DEPENDENCY file1
   DEPENDS file2 file3
   )

Now this produces dependencies on all three files (file{1,2,3}), correct?

Can I make a CUSTOM_COMMAND that has only a lone MAIN_DEPENDENCY?

ADD_CUSTOM_COMMAND(
   OUTPUT ..
   COMMAND ..
   ARGS ..
   MAIN_DEPENDENCY file1
   )

And what happens when I include file1 in both MAIN_DEPENDENCY and DEPENDS?

ADD_CUSTOM_COMMAND(
   OUTPUT ..
   COMMAND ..
   ARGS ..
   MAIN_DEPENDENCY file1
   DEPENDS file1 file2 file3
   )

Thanks,
James