Calculating time differences based on a group

[#1]

Hello,

I have a table with about 60000 records which look something like this:

Table1:
LOAD *
INLINE [
    Group, Start_datetime, Stop_datetime
    "A", "2012-05-01 12:00:01", "2012-05-01 12:15:11",
    "B", "2012-05-01 13:00:01", "2012-05-01 13:15:11",
    "A", "2012-05-01 16:00:01", "2012-05-01 17:13:54",
    "A", "2012-05-02 05:00:01", "2012-05-02 12:15:11",
    "B", "2012-05-02 12:00:01", "2012-05-02 13:15:11",
    "C", "2012-05-02 13:00:01", "2012-05-02 14:15:11"
];

Now I would like to calculate the difference in time between Start_datetime and previous Start_datetime, grouped by the column Group. The result would be something like this:

Group	Start_datetime	Stop_datetime	Time since last one
A	2012-05-01  12:00:01	2012-05-01  12:15:11	n/a
A	2012-05-01  16:00:01	2012-05-01  17:13:54	04:00:00
A	2012-05-02  05:00:01	2012-05-02  12:15:11	13:00:00
B	2012-05-01  13:00:01	2012-05-01  13:15:11	n/a
B	2012-05-02  12:00:01	2012-05-02  13:15:11	23:00:00
C	2012-05-02  13:00:01	2012-05-02  14:15:11	n/a

The next step would be to calculate the sum of all "time since last one" over a time dimension (year) and group.

The problem is not to calculate the time difference. I found the functions previous() and peak() for addressing the previous line, and interval() and timestamp() to calculate the difference and display it in any format. However - I can not find out a way to group the lines by the column Group and calculate only the matching lines!

If you could present me a solution or a hint thereof, I would be grateful.

Peter