Consider the following four specifications to insert an element val into a list l i st. For e ach pair of specifications, state whether one is substitutable for the other. Defend your answer.

(a) require: val is not in list ensure: list contains all of its previous values plus val

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b) ensure: if val is already in l i st, then list is unchanged; otherwise l ist contains all of its previous values plus v a l

(c ) ensure: 1 is t contains all of its previous values plus val

(d) require: list is sorted ensure: l i st contains all of its previous values plus val , and l ist is sorted