# File lib/rspec/matchers/built_in/yield.rb, line 51
        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