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<p>
Basic functor interfaces.
</p>
<p>
Implementors are encouraged, but not strictly required, to make each functor
implementation {@link java.io.Serializable Serializable}.
</p>
<p>
Note that each functor interface extends the 
{@link java.lang.Object#equals Object.equals} contract to state that
<code>equals</code> can return <tt>true</tt> <i>only</i> if 
the specified Object implements the same functor interface 
and is known to produce the same results and/or side-effects 
for the same arguments (if any).  Note that the default
<code>Object.equals</code> implementation 
does in fact adhere to the functor <code>equals</code> contract.
</p>
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