Mappings provide a general protocol for associating the elements of a set of keys each to a value object. A Dictionary is essentially a Set of these Associations, but they are generally used with symbols as keys.
Mapping defines the general protocol at: and at:put: that Sequences use, which also happen to be Mappings. Mappings also support iteration protocols such as keysDo:, valuesDo:, and keysAndValuesDo:.