code: protected <O> O executeTx(TxCallable<O> c) throws Exception {
          return executeTx(c, null);
      }
docs: <p>executeTx.</p>
  
  @param c   a {@link ameba.db.ebean.support.ModelResourceStructure.TxCallable} object.
  @param <O> a O object.
  @return a O object.
  @throws java.lang.Exception if any.