KTDeque.Coq_ESourceLevel-tagged element. At level 0, isomorphic to \'a; at level l > 0, a balanced tree of 2^l base values.
to_list e flattens a level-l element to a list of 2^l base values, in left-to-right order. For a level-0 element built with base x, returns [x].
base x wraps a base value as a level-0 element. This is the one constructor user code needs.
Internal: combine two level-l elements into a level-(l+1) element. Used by the cascade machinery.