not.rb

module Nottable
  class Notter < BasicObject
    attr_reader :orig
    def initialize(orig)
      @orig = orig
    end

    def equal?(other)
      ! orig.equal? other
    end

    def _boolean_methods
      orig.methods.select {|m| m =~ /\?$/}    
    end

    def method_missing(method, *args, &block)
      return super unless _boolean_methods.include? method
      ! orig.send method, *args, &block
    end
  end

  def not  
    Notter.new self
  end  
end  

String.include Nottable

''.nil?
# => false
''.not.nil?
# => true

''.respond_to? :nil?
# => true
''.not.respond_to? :nil?
# => false