$this->keyval[$sub_template_name] .= template_run($this->sub_templates[$sub_template_name], $this->keyval);
# after running a sub-template, clear its sub-templates
$this->keyval[$sub_template_name] .= template_run($this->sub_templates[$sub_template_name], $this->keyval);
# after running a sub-template, clear its sub-templates