It should not be possible to store another object than a parser in PPCompositeParser's subclasses instance variable

[juliendelplanque]

For instance variables that are meant to be used in production.

Allowing that can cause bug that are really complicated to find out and lead to strange behaviour (test being green when run alone and red when run with all the tests of the test suite because the parser is cached in a test resource).

On one of my personal project, I was able to solve this issue using TypedSlot. Maybe we need, in the same direction, to create a special slot to use for PPCompositeParser's subclasses instance variables.

I attached an example of such bad use of the framework to this issue.

PetitParser-Issue50.st.zip

[juliendelplanque]

Maybe a first step to address this problem is to create a code critic rule that checks that no sub-parser instance variable is written.