biojulep: iterators or fold for structures

[timholy]

This package has utilities like collectresidues and collectatoms. Because this allocates storage, it might be of interest to implement an alternative. I see two main options:

• an iterator interface, e.g., EachResidue(struc) or EachAtom(struc). collectresidues could then be implemented as collectresidues(struc) = collect(EachResidue(struc)) but one could also perform other forms of processing (for r in EachResidue(struc) ... end).
• a transducers interface, where foldxl is the core operation.

I have less personal experience with the latter, so left to my own devices I'd probably be tempted to implement the iterator version. But collectresidues/collectatoms are currently probably special cases of foldxl so generalizing them would be perfectly viable too.

[jgreener64]

Sounds like a good idea, and I am also more familiar with the iterator interface.