code: function Identifier(d, s, c){
      this._d = d;
      this._s = s;
      this._c = c;
  }
docs: /*!
  \class Identifier
  \brief Unique and immutable identifier composed of digit, sources, counters
  \param d the digit (position in dense space)
  \param s the list of sources
  \param c the list of counters