This is a very simple ADT representing the Pico
environments system. Environments are represented as abstract grammar components
with a DCT tag. They are linked together
via their third component, which is simply a pointer to the environment that defines
'one scope higher'. As one might expect, there is one operation in the ADT for
creating a new environment (add), one
for assigning the value of an environment (set)
and one for looking up a name in a chain of linked environments (get).