* Refactor.
* More rdoc comments.
! #replace allowed nils in passed array.
* More rdoc comments.
* Refactor.
* Refactor.
- BoundaryError.

An exception that should be raised when #_left_boundary or #_right_boundary
can't calculate boundary because array is empty, really should just be a
RuntimeError. Because it can only be raised as a result of a bug.