Tree structure of the variable populated by the BNF engine
for #matching(variable):

* variable
  |
  +- rules[]: list of production rules (signatures only),
  |
  +- areas[]: table of positions in the input text, the
    |         key is worth the position P; no item value
    +- begin[]: (optional) table for every rule starting
    |  |       at the position P, the key being worth the
    |  |       ending position Pf (decreasing order)
    |  +- []: list of all rules matching [P, Pf] exactly,
    |
    +- end[]: (facultative) table for every rule ending at
      |       the position P, the key being worth the
      |       starting position Pi (increasing order)
      +- []: list of all rules matching [Pi, P] exactly,