I have noted that in sorted collections like the Map and Set modules
(but also in SML/NJ's ORD_SET and ORD_MAP) it is impossible to find the
previous or next element for a given element. This seems strange, given
that the keys in these collections are sorted.

In my particular application I like to store (nested) intervals in a
map, sorted by their starting points. Given a point in an interval, I'd
like to obtain it from a sorted collection. This requires either a
find_less_equal function, that returns the key for an element if it
exists or its predecessor, or a prev_key function, that returns a key's
predecessor such that I can use the regular find afterwards.

Is there a way to implement this in OCaml without making a copy of the
Map module?

-- Christian

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