This is intended to be used as a mixin, usualy with some kind of library that parses optional command line arguments. The feature here is that the options may contain tags ('example_tag:') that are not resolved until they are used. This means that one option may refer to another. An example might be: --stage /tmp/fred --logfile stage:/my.log which would later provide a method 'logfile' that would return '/tmp/fred/my.log'