strange error with strange structure

[sparkane]

I have a collection of arrays, which I refer to thusly:

mCol([numeric_idx])(array_idx)

This allows me to read the expected values no problem. However, when I
try to do something like

mCol(Position)(array_idx_1) = _
mCol(Position)(array_idx_2)

or

mCol(Position)(array_idx_1) = 0

The values don't change.

Is a collection of arrays going to be read-only, or is there something
I'm missing?

spark