session: metas can be added

  - the symbols that appear in the metas are identified in the xml by
    their position in the task:
    - in which declaration
    - in which definition (if that apply otherwise -1)
    - in which constructor(or case in inductive predicate) (if that apply otherwise -1)
    - in which field (if that apply otherwise -1)

  - the md5sum of the prefix of the task that end with the declaration is used to know if the
    symbol have been changed, and if it is obsolete.

  - currently metas that contains obsolete symbol are removed.