Unsually, the final HTML code of the website is composed of several smaller templates containing smaller pieces of it. There are many techniques of combining them. In PHP template engines, the most popular are two forms: manual execution of the templates from the script side and the include directive. Similar solution can be found in OPT 1.x, however in the new version, it should be forgotten - unless we use the quirks mode, the parser does not let us to call the parse() method more than once. In the beginning, I planned to replace it by extending inclusion with "sequences" - a groups of templates to include with their own variables and cache settings. However, one day I was asked on my blog by Coldpeer, whether OPT will support template inheritance. I googled for a while, I looked at it and thought it might be cool. That's how the implementation began.