This module defines the components of trees, graphs
where edges have an in-degree that of 1. The
types defined in this module are concrete implementations
that inherit from the abstract types in the
abstract trees module.

Depending on the concrete subclass, edges in trees either
have lengths formatted as integers or lengths formatted as
floating point numbers.
A concrete node implementation.
A concrete edge implementation, with float length.
A concrete root edge implementation, with float length.
A concrete edge implementation, with int length.
A concrete root edge implementation, with int length.
A concrete tree implementation, with floating point edge lengths.
A concrete tree implementation, with integer edge lengths.