• Combiners use push model instead of pull model - see Combiner Interface in Boost docs
• Combiners are template template parameters - see #2
• Boost.Signals2 default combiner returns boost::optional<T> - in FastSignals, it returns std::optional<T>
• Slot return type never ignored for sig

sigemptyset = _wrap(libc.sigemptyset, [0])
sigemptyset.__doc__ = 'int sigemptyset(sigset_t *set)'

This could be:

sigemptyset = _wrap(
    libc.sigemptyset, [0], 'int sigemptyset(sigset_t *set)')