Hi, I have the following zoo object, which has an index of {0,4}.  The
values are 1 to 20 each day.  In a sense, I have 4 subsets of 5 vectors
index by time.  I want to do a running sum for each subset of data in this
time series as show below in results column.  If I use any of the
rolling/running functions, I get running sum of the whole time series
without subset blocks.  Please advise the best way to approach this
problem.  Thank you, Wilfred

Date    index    [,1]    result
9/8/2010    0    1    1
9/9/2010    1    2    3
9/10/2010    2    3    6
9/11/2010    3    4    10
9/12/2010    4    5    15
9/13/2010    0    6    6
9/14/2010    1    7    13
9/15/2010    2    8    21
9/16/2010    3    9    30
9/17/2010    4    10    40
9/18/2010    0    11    11
9/19/2010    1    12    23
9/20/2010    2    13    36
9/21/2010    3    14    50
9/22/2010    4    15    65
9/23/2010    0    16    16
9/24/2010    1    17    33
9/25/2010    2    18    51
9/26/2010    3    19    70
9/27/2010    4    20    90

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