Specify "ordered sets" to simplify synthax about calliope

[brynpickering]

I'm not keen to add all this extra core syntax math - the helper functions are there to extend the math in a way that remains modular. Although they are wordy, they are relatively explicit (they can certainly be given better names!). "<<" and ">>" are creating a whole new syntax, and "[-1]" and "[0]" overlap with existing slicing syntax which will make them difficult to parse.

You're right that those two helpers are specific to ordered sets and we have no way of controlling for someone applying them in the wrong context. We could have some check in the helpers for either user-defined metadata (ordered: true) or it being a datetime/numeric dtype. As a user, you could feasibly ensure the ordering of your sets by reorganising the input data dimensions, we just need to provide an API for that.

[irm-codebase]

Defining ordered sets in theory should also allow us to perform more complicated summations with conditionals (like "sum new capacity installed for all investsteps lower than this one").

I've yet to land on an approach that is easy to read for these cases, though...

[irm-codebase]

I'm fine with not implementing the extra syntax :)
However, a personal wish is to use ordered sets for numeric comparisons. Kind of: sum over this dimension while it is lower than [some position]. Would be a god send for pathways math.

I will close this issue as "not planned", and rewrite another with your feedback in mind.