Clarify the role of part vs. score vs single part score

[hanshoglund]

We often use single-part scores in the implementation. That is scores which have an informal guarantee to have no overlapping events.

• We usually want to manipulate things which have both offset and duration (i.e. scores and voices). Often we need to work on single voices.
• Voice is great because it enforces no overlaps in its type
• However it is inconvenient not to have the offsets around, forcing us to do accumulation over time.
• Maybe add a newtype wrapper for score with the static guarantee, that is, a hidden implementation and safe constructors Part (Maybe a) -> SingleVoiceScore a, SingleVoiceScore a -> Voice (Maybe a).

[hanshoglund]

Single-voice scores should be factored out. We should not think about the implementation of scores, i.e.

• "It is great to use Score, because then we have the absolute onsets around"

Instead, think semantically:

• Score a is a sequence of possibly overlapping notes
• Voice a is a sequence of non-overlapping notes
• Voice (Maybe a) is a sequence of non-overlapping notes or rests, and is isomorphic to [Either Duration (Voice a)]