Recursive rule definitions

[Dane Finlay]

JSGF spec sections 4.8-4.9 discuss right recursive rules:

<command> = <action> | (<action> and <command>);
<action> = stop | start | pause | resume | finish;

and nested right recursive rules:

<X> = something | <Y>;
<Y> = another thing <X>;

While constructing or parsing rules like these works, matching speech to them doesn't. Matching will fail with a "maximum recursion depth exceeded" error.

The JSGF spec has some notes on how to support right-recursive rules:

Any right recursive rule can be re-written using the Kleene star *' and/or the plus operator +'. For example, the following rule definitions are equivalent:

   <command> = <action> | (<action> and <command>);
   <command> = <action> (and <action>) *;

Although it is possible to re-write right recursive grammars using the +' and *' operators, the recursive form is permitted because it allows simpler and more elegant representations of some grammars. Other forms of recursion (left recursion, embedded recursion) are not supported because the re-write condition cannot be guaranteed.

With that in mind, it should be possible to use rewritten matcher elements somehow when right recursion is detected.