t its default of ``-1``, the size
 is unbounded, and a plain Python dictionary is used to
 relate URI strings to :class:`.Template` instances.
 Otherwise, a least-recently-used cache object is used which
 will maintain the size of the collection approximately to
 the number given.

:param filesystem_checks: When at its default value of ``True``,
 each call to :meth:`.TemplateLookup.get_template()` will
 compare the filesystem last modified time to the time in
 which an existing :class:`.Template` object was created.
 This allows the :class:`.TemplateLookup` to regenerate a
 new :class:`.Template` whenever the original source has
 been updated. Set this to ``False`` for a very minor
 performance increase.

:param modulename_callable: A callable which, when present,
 is passed the path of the source file as well as the
 requested URI, and then returns the full path of the
 generated Python module file. This is used to inject
 alternate schemes for Python module location. If left at
 its default of ``None``, the built in system of generation
 based on ``module_directory`` plus ``uri`` is used.

All other keyword parameters available for
:class:`.Template` are mirrored here. When new
:class:`.Template` objects are created, the keywords
established with this :class:`.TemplateLookup` are passed on
to each new :class:`.Template`.

Nc