See the attached and very rought PEP.

Basically if you want to see the idea, read section
2.5.1 at:

ftp://ftp.ps.uni-sb.de/pub/papers/ProgrammingSysLab/TOPLAS99.ps.gz

The abstract of that paper is at:

http://www.mozart-oz.org/papers/abstracts/TOPLAS99.html

The abstarct of the PEP is as follows:

    Stackless Python [1] is an ideal environment within
which to
    implement logic variables for thread
synchroniziation [2].
    The 3 additions required for this are 1) the
addition of the
    "unknown" keyword that declares a logic variable,
2) the
    "final" keyword that declares a logic variable to
be single-
    assignment and 3) the "=:=" operator that performs a 
    unification of a logic variable with an expression.