Newbie help / Grouping and sum

In XSLT 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>
                        <SumItem>
                                   <SumItemResult>
                                               <AggSetSumItem>

<Days>1</Days>

<Service>TE</Service>
                                               </AggSetSumItem>
                                               <Charge>

<Amount>240.00</Amount>
                                               </Charge>
                                   </SumItemResult>
                        </SumItem>
                        <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>
                        <SumItem>
                                   <SumItemResult>
                                               <AggSetSumItem>

<Days>1</Days>

<Service>TT</Service>
                                               </AggSetSumItem>
                                               <Charge>

<Amount>350.20</Amount>
                                               </Charge>
                                   </SumItemResult>
                        </SumItem>
                        <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
2 350.70

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

Any help is appreciated.

10x in advance.

Plamen