  We provide sufficient conditions for a free amalgamated product of
torsionfree nilpotent groups to be residually nilpotent. We also characterise
the residual nilpotence of certain higher-dimensional amalgams of unipotent
groups over the rationals (known as KMS groups) in terms of their defining
Cartan matrix. As an application, we give a normal form for the elements of a
minimal Kac-Moody group over the rationals.
