Class of values that can be reversed (retrograded).

For positioned values succh as Note, the value is reversed relative to its middle point, i.e. the onset value becomes the offset value and vice versa.

For non-positioned values such as Stretched, the value is reversed in-place.

FIXME Second law is incompatible with revDefault (and the Span definition below)

Law

rev (rev a) = a

abs (x^.duration) = (rev x)^.duration

rev s transform a = rev (s transform a)

or equivalently,

transform . rev = fmap rev . transform

For Span

rev = over onsetAndOffset swap

View the reverse of a value.

>>> [1,2,3] & reversed %~ Data.List.sort
[3,2,1]

A default implementation of rev