ndindex is a library that allows representing and manipulating objects that can
be valid indices to numpy arrays, i.e., slices, integers, ellipses, None,
integer and boolean arrays, and tuples thereof. The goals of the library are:
- Provide a uniform API to manipulate these objects. Unlike the standard index
  objects themselves like slice, int, and tuple, which do not share any methods
  in common related to being indices, ndindex classes can all be manipulated
  uniformly. For example, idx.args always gives the arguments used to construct
  idx.
- Give 100% correct semantics as defined by numpy's ndarray. This means that
  ndindex will not make a transformation on an index object unless it is correct
  for all possible input array shapes. The only exception to this rule is that
  ndindex assumes that any given index will not raise IndexError (for instance,
  from an out of bounds integer index or from too few dimensions). For those
  operations where the array shape is known, there is a reduce() method to
  reduce an index to a simpler index that is equivalent for the given shape.
- Enable useful transformation and manipulation functions on index objects.

WWW: https://github.com/Quansight-Labs/ndindex