# Implementation of RFC 1982 Serial Number Arithmetic.

Run as a program, this takes start and finish serial values and tells
you what intermediate steps (if any) are needed to make the specified
change.

Used as a module, this implements the `Serial` class and an iterator
which returns the intermediate steps (if any) needed between two
`Serial` values.

Per the RFC, only two operations are defined on `Serial` objects:
addition and comparision, both within a restricted range specified
by the RFC.

The default modulus is `2**32`, you can change this by subclassing the
`Serial` and overriding the class's `modulus` variable.  The modulus
must be a power of two.

See RFC 1982 for discussion of the ways in which serial numbers do not
work like normal integers.  In particular, note that there's a corner
case in which one can have a pair of serial numbers `I1` and `I2`
where `I1` is neither equal to, less than, nor greater than `I2`.
This is deliberate and is not a bug in the code.  See the RFC.

The `find_intermediate()` iterator takes two `Serial` values `start`
and `finish` and yields the sequence of intermediate values (if any)
needed to step from `start` to `finish`.

There are also a couple of test programs that use `random.randint()`
to generate test values.  Maybe someday I'll recode with `unittest`.