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Specification of the Dependency ADT.

The Dependency ADT shall be implemented in Java, and will be tested
using Sun's Java 2 Runtime Environment, Standard Edition, version 1.6.0.
The code for this implementation will be in the default package, and
shall define a public class named Dependency.  That class shall
provide the following public static methods:

Signature:

    Dependency.create : Refreshable * Refreshable -> Dependency
    Dependency.first  : Dependency                -> Refreshable
    Dependency.second : Dependency                -> Refreshable
    Dependency.isOkay : Dependency                -> boolean

    Dependency.first (Dependency.create (r1, r2)) = r1

    Dependency.second (Dependency.create (r1, r2)) = r2

    Dependency.isOkay (Dependency.create (r1, r2))
        =  Refreshable.era(r1) <= Refreshable.era(r2)

Values of the Dependency ADT shall also implement the public
dynamic methods equals(Object) and hashCode() such that

    If v1 and v2 are values of the Dependency ADT, then

        v1.equals(v2) if and only if
            Dependency.first(v1).equals(Dependency.first(v2))
        and Dependency.second(v1).equals(Dependency.second(v2))

    If v1 is a value of the Dependency ADT, but x is not, then

        v1.equals(x) returns false.

    If v1 and v2 are values of the Dependency ADT, and

        v1.equals(v2)

    then v1.hashCode() == v2.hashCode().

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