I am sure there is a straightforward way to do this, but somehow I  can't
seem to get it to work.  I want to sum multiple cells in a 2d array, based
off the x and y values in a given column.  As a simplified example, use the
data:
       A    B    C    D

1     568 783 999 9091
2     200  12  14    12
3     100  11  9      0
4     100  0    1      4
5     390  2    5.5   6

My problem, then, is that I wish to have a formula which sums all cells in
the range B2:D5 for which column "A" has the number 100 and row 1 has the
number 999 (that is, the answer here should be 10).  Of course the real
spreadsheet is much larger than this, but this example should suffice

thanks.