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| A read-only map abstraction that does not permit modifications. A mapping abstraction from elements of type K to elements of type E A map stores an element of type E under a key of type K. Each key can hold one element.
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Design note: It would also be reasonable to have a $RO_MAP be a subtype of $CONTAINER{TUP{K,E}}. However, since we are using the array notation, to avoid confusion with the way arrays behave we view maps as containers of the target elements, with the keys being a kind of indexing method. Arrays are then a special kind of map where the keys are integers. Though we don't make use of this possibility in this library, the use of array notation is consistent with it.
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