[Ruby] RSpecのexpectを読んでいたら ref: http://qiita.com/kidachi_/items/a0417ba68ce38382f68d

file1.rb

def expect(*target, &target_block)

file2.rb

  raise ArgumentError.new("You must pass an argument or a block to #expect but not both.") unless target.size == 1

syntax.rb

  def expect(*target, &target_block)
    target << target_block if block_given?
    raise ArgumentError.new("You must pass an argument or a block to #expect but not both.") unless target.size == 1
    ::RSpec::Expectations::ExpectationTarget.new(target.first)
  end