CQL::Parser provides a mechanism to parse Common Query Language (CQL)
statements. The best description of CQL comes from the CQL homepage at the
Library of Congress http://www.loc.gov/z3950/agency/zing/cql/

CQL is a formal language for representing queries to information retrieval
systems such as web indexes, bibliographic catalogs and museum collection
information. The CQL design objective is that queries be human readable
and human writable, and that the language be intuitive while maintaining
the expressiveness of more complex languages.

A CQL statement can be as simple as a single keyword, or as complicated as
a set of compoenents indicating search indexes, relations, relational
modifiers, proximity clauses and boolean logic. CQL::Parser will parse CQL
statements and return the root node for a tree of nodes which describes
the CQL statement. This data structure can then be used by a client
application to analyze the statement, and possibly turn it into a query
for a local repository.