gistfile1.rb

#!/usr/bin/env ruby

class Usually
  undef_method *instance_methods.grep(/^(?!__|object_id)/)

  def initialize(usual)
    @usual = usual
  end

  def unless(cond, &unusual)
    if cond
      unusual.call
    else
      @usual.call
    end
  end
end

def usually(&block)
  Usually.new(block)
end

usually do
  p ARGV.inject('I will work with ') { |msg, arg|  msg << "#{arg} "}
end.unless ARGV.empty? do
  p 'Provide at least one argument, please.'
end

# ./usually.rb ciao miao bau =>  "I will work with ciao miao bau "
# ./usually.rb               =>  "Provide at least one argument, please."