class Object

def in?(another_object)

to +#include?+.
This will throw an +ArgumentError+ if the argument doesn't respond

"Konata".in?(characters) # => true
characters = ["Konata", "Kagami", "Tsukasa"]

any object which responds to +#include?+. Usage:
Returns true if this object is included in the argument. Argument must be 
def in?(another_object)
  another_object.include?(self)
rescue NoMethodError
  raise ArgumentError.new("The parameter passed to #in? must respond to #include?")
end