Models and classes to supplement the stdlib 'collections' module.

Highlights include:

  * RangeMap: A mapping that accepts a range of values for keys.
  * Projection: A subset over an existing mapping.
  * KeyTransformingDict: Generalized mapping with keys transformed by a
    function.
  * FoldedCaseKeyedDict: A dict whose string keys are case-insensitive.
  * BijectiveMap: A map where keys map to values and values back to their keys.
  * ItemsAsAttributes: A mapping mix-in exposing items as attributes.
  * IdentityOverrideMap: A map whose keys map by default to themselves unless
    overridden.
  * FrozenDict: A hashable, immutable map.
  * Enumeration: An object whose keys are enumerated.
  * Everything: A container that contains all things.
  * Least, Greatest: Objects that are always less than or greater than any
    other.
  * pop_all: Return all items from the mutable sequence and remove them from
    that sequence.
  * DictStack: A stack of dicts, great for sharing scopes.
  * WeightedLookup: A specialized RangeMap for selecting an item by weights.