@jaberg Unfortunately, it's not so clear-cut that nodes are always either sources or sinks; very often they are both, and in many cases they are time dependent, and in others they are not. So basically we want to be able to support all these possible cases:

1. No input, provides non-time-dependent output
2. No input, provides time-dependent output
3. Accepts input, provides non-time-dependent output
4. Accepts input, provides time-dependent output

So, whether the node accepts input doesn't imply that time won't be used. Can you think of a syntax for supporting all those cases unambiguously?

@drasmuss I'm not sure it would really add that much overhead... we can provide helper functions in `nengo.helpers` for the common cases we use in examples / teaching; already we have `white_noise` and `piecewise`, we could add in `sine`. I'm not sure what else we use for simple examples?