select()ing a Map

[Ilya Zakharevich <nospam-abuse@ilyaz.org>]

I see that there is a request for a specification what should select()
do on a Map.

• In case when the second argument is a Map and flag is 0, the non-0
  values returned by the function f should be either all scalars, or
  all the same non-scalar type.

• In this case, the type of the non-0 values returned by f()
  determines the type returned by select() — when this makes sense.
  (Returning a scalar non-0 value is equivalent to returning a non-0
  Map.)

Hope this helps,
Ilya