This script displays the structure of loops and conditional statements in
an HTML::Template::Pro template, and is an aid for debugging errors
reported by the xt/author/valid-templates.t test. It also identifies the
following errors:
* TMPL_IF/TMPL_UNLESS/TMPL_LOOP with no closing tag
* TMPL_ELSE with no initial TMPL_IF or TMPL_UNLESS
* extra closing tags
* HTML comment of the form <!-- TMPL_FOO ..., where TMPL_FOO is not a valid HTML::Template::Pro tag