----------------------------------------------------------------------
HostMatcher
----------------------------------------------------------------------

A platform identifier looks something like:

   OS=Linux,   ISA=i386
   OS=Windows, ISA=PowerPC
   
The HostMatcher takes the following inputs:

  1. A list of "available" platform identifiers.
  2. A list of transformation rules to describe how different
     platforms are compatible with each other.
  3. A single "requested" platform identifier.

And produces the following output:

  One of the platform identifiers from the "available" list that
  is compatible with the "requested" platform identifier.

--------------------

"Test.py" lets you try different combinations of inputs to see how
the program operates.

--------------------

"ExecutableSelector.py" is designed for the following situation:

  - You have to run the same program on multiple platforms.
  - You have a version of the executable for each target platform.
  - You need to figure out, automatically, which of the executables
    to use on the current platform (for example, in a Makefile).

Given a list of executables, "ExecutableSelector.py" will try to
find the appropriate executable for the given platform and execute
it.  The executable file names should be of the form:

   xxx.{OS}-{ISA}