The current implementation of step and impulse responses for MIMO systems requires the specification of an input channel, so that you always get a SIMO (or SISO) time response. As pointed out in #453, this is not symmetric with the frequency responses, where you can index the input and output channels. There is no reason this couldn't be done consistently for step and impulse responses by just iterating over the set of all input channels. This would help make time and frequency responses more consistent.

Note that one inconsistency will remain: when you compute a forced response, you have to specify the particular input. So in that case you can't index the input channel, only the output channel.