`method_defined?` and `method_implemented?` are exactly the same.  Initially, this was a little weird to me but I get why: w/o having an instance of the class/module, you can't use `respond_to?`, and you don't have a reference to an instance...so this is the best you can do.  I think a comment would be good here to explain that, and this seems like a good use case for `alias`.  What do you think about this?

``` ruby
def method_defined?(m)
  m.method_defined?(@method_name)
end

# Ideally, we'd use `respond_to?` for `method_implemented?` but we need a reference to an
# instance to do that and we don't have one.  Note that we may get false negatives:
# if the method is implemented via `method_missing`, we'll return `false` even though it
# meets our definition of "implemented". However, it's the best we can do.
alias method_implemented? method_defined?
```