The type for keys.

new_key n is a new key with name k.

The type for context maps.

empty is the empty context.

add k v t is t augmented with the binding (k, v). Any previous binding of k is removed.

mem k t is true iff k has been added to t.

find k t is v is the binding (k, v) has been added to t, otherwise it is None.

merge ~default t merges t on top of default. If a key appears in both default and t, the value present in t is kept.

dump dumps the state of t.