code: public int restrict(final int r, final int var) {
      final int res;
      if (var < 2)
        return r;
      varset2svartable(var);
      initRef();
      res = restrictRec(r, (var << 3) | CACHEID_RESTRICT);
      return res;
    }
docs: Restricts the variables in the BDD {@code r} to constants true or false.  The restriction is submitted in the BDD
  {@code var}.
  @param r   the BDD to be restricted
  @param var the variable mapping as a BDD
  @return the restricted BDD