I wanted to call it dependentEncoder since it exhibits a bit of context dependent parsing, but then there's the dependent sum aspect and the name would get confusing. While a dependent sum encoder must be ctx dependent, the converse is not true. We can get a simpler

  => Encoder check parse a a'
  -> (a -> Encoder Identity parse b b')
  -> Encoder check parse (a,b) (a', b')

by wrapping/unwraping via Const. I'm tempted to add such a util for convenience but am not coming up with a good name. Maybe contextDependentEncoder on account of it being the simpler such encoder?

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