adaptArrayPattern = (optionalPatt, length) =>
    harden(optionalPatt.slice(0, length).map(patt => MM.opt(patt)))
Optional specimen elements which are `undefined` pass unconditionally.
We encode this with the `M.or` pattern so it also produces a good
compression distinguishing `undefined` from absence.

@param {Pattern[]} optionalPatt
@param {number} length
@returns {Pattern[]} The partialPatt