Cannot install ruby gem in OSX

Developers Ruby API
13

I very much like the way they (the binder) wrapped the library class within a Ruby mixin module.
It is actually meant to be mixed into the programmer’s namespaces:

module AuthorToplevelNamespace
  module ThisPlugin

    require 'clipper' # assumes gem was pre-installed
        include Clipper # includes the Clipper mixin module

        # Now the local constant Clipper is a proxy lookup
        # to the ::Clipper::Clipper class.(Because identifiers 
        # are constants and get mixed in during the include.)

        def self::union_method(
          a = [[0, 0], [0, 100], [100, 100], [100, 0]] ,
          b = [[-5, 50], [200, 50], [100, 5]]
        )
          c = Clipper::new()
          c.add_subject_polygon(a)
          c.add_clip_polygon(b)
          c.union( :non_zero, :non_zero )
        end
 
  end
end