code: public Formula importFormula(final Formula formula) {
      if (this.importer == null) {
        this.importer = new FormulaFactoryImporter(this);
      }
      final Formula imported = formula.transform(this.importer);
      adjustCounters(imported);
      return imported;
    }
docs: Imports a formula from another formula factory into this factory and returns it.  If the current factory of the
  formula is already this formula factory, the same instance will be returned.
  @param formula the formula to import
  @return the imported formula on this factory