In the sketch of proof of lemma (1.1) replace
"can be written as a product of N unipotent triangular matrices ..."
with
"can be written as a product of N unipotent triangular matrices, the i^th one being lower triangular when i is even, upper triangular when i is odd, ..."
This mistake is mine; the original proof of Dennis did specify the shapes.

Wilberd van der Kallen