Happy to add comment to clarify. I think that the `try ... except` holds water here. 

Note that the `coord_from_term` is using results derived by the PyKE rules and that there are two possible exceptions. So for me, `try ... except` is succinct and the right tool for the job.

Yes, the overall point is that for hybrid pressure there are two possible formula terms:
* `p0: var1 a: var2 b: var3 ps: var4`, and
* `ap: var1 b: var2 ps: var3`

Where `ap = p0 * a`