  An additive group action on an affine 3 -space over a complex Dedekind domain
A is said to be twin-triangular if it is generated by a locally nilpotent
derivation of A[y,z,t] of the form rd/dy+p(y)d/dz + q(y)d/dt, where r belongs
to A and p,q belong to A[y] . We show that these actions are translations if
and only if they are proper. Our approach avoids the computation of rings of
invariants and focuses more on the nature of geometric quotients for such
actions.
