I see we are actually constructing a "delta" when one didn't exist originally.  It took me a while to grasp that, though, so I think a simple comment explaining it would be worthwhile.
I also wonder if it would be more transparent/honest to test the formula terms than to use the try-block, 
e.g. more like..
```
    if 'ap' in engine.requires['formula_terms']:
        delta = coord_from_term('ap')
    else:
        . . .
```
This seems to what is effectively tested here, though I'm a bit vague on Pyke parts