The problem in designing a "general" tool is deciding what features to provide.

We can try to define features that might be useful, but it is also a good idea to make the list of features open-ended, so that new ones can be easily added when needed.

Features can be added by generalizing or specializing existing ones. For example, we provide segment variables that match zero or more input elements. We can specialize this by providing for a kind of segment variable that matches one or more elements, or for an optional variable that matches zero or one element. Another possibility is to generalize segment variables to specify a match of m ton elements, for any specified m and n. These ideas come from experience with notations for writing regular expressions, as well as from very general heuristics for generalization, such as "consider important special cases" and "zero and one are likely to be important special cases."