Generic extensions of nilpotent $k[T]$-modules, monoids of partitions   and constant terms of Hall polynomials
  We prove that the monoid of generic extensions of finite dimensional nilpotent $k[T]$-modules is isomorphic to the monoid of partitions (with addition of partitions). Moreover we give a combinatorial algorithm that calculates constant terms of classical Hall polynomials. 
Justyna Kosakowska
math.RT