  Recently we have used the Carlitz exponential map to define a finitely
generated submodule of the Carlitz module having the right properties to be a
function field analogue of the group of units in a number field. Similarly, we
constructed a finite module analogous to the class group of a number field.
  In this short note more algebraic constructions of these "unit" and "class"
modules are given and they are related to Ext modules in the category of
shtukas.
