XSLT Grouping and Sum

[Terious]

In XML ver 1.0.

Here is XML I have:

<CustRef>
<Contract>
<ContractId>1</ContractId>
<SumItem>
<SumItemResult>
<AggSetSumItem>
<Days>1</Days>
<Service>TT</Service>
</AggSetSumItem>
<Charge>
<Amount>40.00</Amount>
</Charge>
</SumItemResult>
</SumItem>
<ContractId>1</ContractId>
<SumItem>
<SumItemResult>
<AggSetSumItem>
<Days>1</Days>
<Service>TE</Service>
</AggSetSumItem>
<Charge>
<Amount>240.00</Amount>
</Charge>
</SumItemResult>
</SumItem>
<ContractId>1</ContractId>
<SumItem>
<SumItemResult>
<AggSetSumItem>
<Service>TT</Service>
</AggSetSumItem>
<Charge>
<Amount>880.00</Amount>
</Charge>
</SumItemResult>
</SumItem>
</Contract>
<Contract>
<ContractId>2</ContractId>
<SumItem>
<SumItemResult>
<AggSetSumItem>
<Days>1</Days>
<Service>TT</Service>
</AggSetSumItem>
<Charge>
<Amount>0.50</Amount>
</Charge>
</SumItemResult>
</SumItem>
<ContractId>2</ContractId>
<SumItem>
<SumItemResult>
<AggSetSumItem>
<Days>1</Days>
<Service>TT</Service>
</AggSetSumItem>
<Charge>
<Amount>350.20</Amount>
</Charge>
</SumItemResult>
</SumItem>
<ContractId>2</ContractId>
<SumItem>
<SumItemResult>
<AggSetSumItem>
<Service>TZ</Service>
</AggSetSumItem>
<Charge>
<Amount>880.00</Amount>
</Charge>
</SumItemResult>
</SumItem>
</Contract>
</CustRef>

I want to group by <ContractId> and sum <Amount> where <Service> is 'TT' and <Days> is greater than 0. For example:

1 40
2 400

Using Muenchian method, I can sum the <Amount> for every <ContractId>, but am can't select only these <SumItem> for which <Service> is equal to 'TT' and <Days> is greater than 0.

Any help is appreciated.

10x in advance.

Plamen