code: public boolean hasReceipt( String receipt_id ) {
      synchronized( _receipts ) {
        for (Iterator i=_receipts.iterator(); i.hasNext();) {
          String o = (String)i.next();
          if (o.equals(receipt_id)) return true;
        }
      }
      return false;
    }
docs: Checks to see if a receipt has come in.
  
  @param receipt_id the id of the receipts to find