LongStack formula when imported via XLS

[Jon Elson <elson pico-systems com>]

I've been using the NExS spreadsheet for years, but it looks like
Gnumeric now does everything that program did.  So, I exported
some spreadsheets in xls format into Gnumeric, and the
sheets show fine on the screen.  But, when I click on some of the
formulas, I see      =#"#LongStack!" instead of the expected
formula.  The displayed value, however, appears to be correct!

The formula in NExS format is something like :
= CSUM((D$5..D$78),#{3,0}=="x")
which means if a field 3 places to the right contains "x"
sum the cells from D5 to D78.  This is a bit different
from Excel format, so I suspect NExS translates the
formula before exporting, but, I can't see what it
came up with.

Any idea what this LongStack thing is?

Thanks,

Jon