HTML_Template_Sigma implements Integrated Templates API designed by Ulf
Wendel.

Features:
* Nested blocks. Nesting is controlled by the engine.
* Ability to include files from within template: <!-- INCLUDE -->
* Automatic removal of empty blocks and unknown variables (methods to
manually tweak/override this are also available)
* Methods for runtime addition and replacement of blocks in templates
* Ability to insert simple function calls into templates:
func_uppercase('Hello world!') and to define callback functions for these
* 'Compiled' templates: the engine has to parse a template file using
regular expressions to find all the blocks and variable placeholders. This
is a very "expensive" operation and is an overkill to do on every page
request: templates seldom change on production websites. Thus this feature:
an internal representation of the template structure is saved into a file
and this file gets loaded instead of the source one on subsequent requests
(unless the source changes)
* PHPUnit-based tests to define correct behaviour
* Usage examples for most of the features are available, look in the docs/
directory