PIso

create a PIso using a pair of functions: one to get the target and one to get the source.

create a PIso between any type and itself. id is the zero element of optics composition, for all optics o of type O (e.g. Lens, Iso, Prism, ...): o composeIso Iso.id == o Iso.id composeO o == o (replace composeO by composeLens, composeIso, composePrism, ...)