Best practice: How and where to put protocols and their implementation?

I think it would help to have a common place to put protocol definitions and also implementations. It would be in particular interesting if protocols are implemented for existing modules.

For discussion:

definition:

file: lib/protocols/incredible.ex

implementation for a new modules within the module.

implementation for an existing module (e.g. Integer) in:

file: lib/protocols/impl/integer.ex

or

file: lib/protocols/integer_impl.ex

?