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