Also, generators cannot yield control while other functions are
    executing, unless those functions are themselves expressed as
    generators, and the outer generator is written to yield in response
    to values yielded by the inner generator.  This complicates the
    implementation of even relatively simple use cases like asynchronous
    communications, because calling any functions either requires the
    generator to "block" (i.e. be unable to yield control), or else a
    lot of boilerplate looping code must be added around every needed
    function call.