usage of bands function in rio calc

[perrygeo]

Noticed as I was going through test coverage, we never actually test the rio calc bands function. It's not in the docstring or the documentation. I can't get it to work directly:

$ rio calc "(bands 0)" tests/data/shade.tif /tmp/out.tif
...
AttributeError: 'list' object has no attribute 'shape'

So I'm unclear about the purpose of bands - is it just an internal function? Why is it exposed to the CLI? And how is it meant to be used versus take and read?