>  What do you think about something like RSpec.flat_map(array) or some such (i.e. hanging flat_map off a const rather than call)?

I'm fine with that.  I almost suggested the route of defining one of two classes with a single class method...but a lambda seemed like a lighter-weight way of doing that.  I'm fine either way.

> Another concern here is that converting the block to a proc and back has a cost.