class RSpec::Matchers::BuiltIn::YieldProbe

def assert_used!
  return if @used
  raise 'You must pass the argument yielded to your expect block on ' \
        'to the method-under-test as a block. It acts as a probe that ' \
        'allows the matcher to detect whether or not the method-under-test ' \
        'yields, and, if so, how many times, and what the yielded arguments ' \
        'are.'
end