Grouping and sum values in XI

Hello,

I 'm working with invoice and I have this source structure as XI input:

Invoice

-- |....

-- |....

-- |Item1

-

---

|taxcode=01

-

---

|Amount=12

--|Item2

-

---

|taxcode=08

-

---

|Amount=10

--|Item3

-

---

|taxcode=01

-

---

|Amount=24

Now my scope is to map these fields to the IDOC segment E1EDP04 grouping by taxcode (MWSKZ taxcode)and putting the sum of the group amount (MWSBT amount).

IDOC:

--|...

--|...

--|E1EDP01

-

---

|...

-

---

|...

-

---

|EIEDP04

-

---

|MWSKZ=01

-

---

|MWSBT=36

-

---

|...

--|E1EDP01

-

---

|...

-

---

|...

-

---

|EIEDP04

-

---

|MWSKZ=08

-

---

|MWSBT=10

-

---

|...

How can I group by a field in XI?

Thank you

Corrado