List-like data structures implemented using nested data types and polymorphic recursion. Also called "n-ary random access lists". They supports O(log(n)) lookup while still having amortized O(1) access to the left end of the sequence. Somewhat similar to finger trees, but much simpler, and the ternary and quaternary versions are also more memory efficient; however, modifying the right end of the sequence is still slow. See Data.Nested.Seq for general comments and Data.Nested.Seq.Binary.Lazy for an explanation of the data structure.