Set Abstract Data type in Haskell. Read all parts of this question before
you answer any part of the question so you can make sure all parts of the
code for your Set ADT implementation are consistent.

(a) What is the most essential property of Abstract Data Types in computer
science? Name or clearly and succinctly describe the property.

(b) Code Haskell export controls for a Set abstract data type that gives
a user of the module the ability to create an empty set, insert an
element in a set, delete an element in a set, and test if an element is
a member of a set. Your module MUST obey the essential property of
abstract data types.

(c) Give an appropriate type definition for the your Set abstract data
type assuming an unordered list implementation.

(d) Fill in the Haskell polymorphic types and Haskell code for the Set
abstract data type assuming an unordered list implementation.

Again, your module MUST obey the essential property of abstract data
types.

> empty ::

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> insert ::

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> member ::

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> delete ::

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