eval.c:9586
static VALUE
rb_thread_start(klass, args)
    VALUE klass, args;
{
    if (!rb_block_given_p()) {
	rb_raise(rb_eThreadError, "must be called with a block");
    }
    return rb_thread_start_0(rb_thread_yield, args, rb_thread_alloc(klass));
}
