What problem does this solve? Let's say you have a game in which different events happen. Each event is subject + action. As an example, let the action be "deal 10 damage to you" and the subject is either "You" or "A monster". Now we cannot just simply concatenate the two strings, because of grammar:

• You deal 10 damage to you should be You deal 10 damage to yourself.
• A monster deal 10 damage to you should be A monster deals 10 damage to you.

flaggelation is a tiny DSL that evaluates template strings in which you can set und unset flags and add conditional strings based on the flags that are set.

A template for the example above could be to use You[+p=2] as the you subject and A monster[+p=3] for the monster, setting a flag named p to a value of "2" or "3" for 2nd and 3rd person singular. Then the action string template can look like this: deal[p=3?s] 10 damage to [p=2?yourself|you]. Here the [p=3?s] checks if a p flag was set to value 3 before and if so, inserts the additional s. The [p=2?yourself|you] inserts yourself if the p flag was set to 2 and you otherwise.

• set flag: [+foo] or [+foo=bar] (if specific value is needed)
• unset flag: [-foo] or override it with [+foo=othervalue]
• conditional insert: [foo?insert this if foo is set] or [foo?insert this|otherwise this]
• paste value of flag: {foo}. If [+foo=abc] was set before, this pastes abc, otherwise it pastes foo. {foo|alternative} can be used to paste an alternative string if foo is not set. {foo|} pastes the value of foo only if foo is set.

currently nested expressions are not supported and some of the syntax tokens cannot be used in the free text of the template string.