code: function ProductionRule(lhs, rhs, head) {
    this.lhs = lhs;
    this.rhs = rhs;
    this.head = head;
    // feature structure of the constraints specified with the rule
    this.fs = null;
  }
docs: Constructor - lhs is a string - rhs is an array of strings - head is a number pointing to a rhs nonterminal