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).