This module provides a collection of decorators that makes it easy to write
software using contracts.

Contracts are a debugging and verification tool. They are declarative statements
about what states a program must be in to be considered "correct" at runtime.
They are similar to assertions, and are verified automatically at various
well-defined points in the program. Contracts can be specified on functions and
on classes.

Contracts serve as a form of documentation and a way of formally specifying
program behavior. Good practice often includes writing all of the contracts
first, with these contract specifying the exact expected state before and after
each function or method call and the things that should always be true for a
given class of object.

Contracts consist of two parts: a description and a condition. The description
is simply a human-readable string that describes what the contract is testing,
while the condition is a single function that tests that condition. The
condition is executed automatically and passed certain arguments (which vary
depending on the type of contract), and must return a boolean value: True if the
condition has been met, and False otherwise.
