Nested Transaction

Hi,
I have two function as below

[Transaction(TransactionPropagation.Required, ReadOnly = false)]
public bool AddAndDeleteMultipleFiles(parameters)
{
foreach (int i = 0;i<20;i++)
{
bool a = AddAndDeleteSingleFile(parameteres);
}
}
[Transaction(TransactionPropagation.Required, ReadOnly = false)]
public bool AddAndDeleteSingleFile(parmateres)
{

}
Let's say I have call AddAndDeleteSingleFile for 20 files. and I want to roll back only those files which failed to commit in AddAndDeleteSingleFile function.How can we do this in spring.net?