Implements a specific map from prefixes to namespace URI's. Prefixes
will automatically be generated and a record maintained of the
namespace URI associated with it. One may also specify an explicit prefix
to namespace URI mapping to use.
Returns a prefix that can be used for the given namespaceURI. If a
previous mapping did not exist then a new prefix is created for this
namespace URI and returned.
Parameters:
namespaceURI - The namespace URI for which a prefix is required
Returns:
String The prefix to be used for the specified namespace URI