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Rewrite the definitions of second/2 and tail/2
above to include explicit calls to the unification operator =/2.
![\begin{solution}\texttt{second/2}
\begin{verbatim}
second([_, Second\vert _], Result):-
Result = Second.\end{verbatim}\end{solution}](img15.png)
Define a predicate fifth/2 that, given a list as first argument,
returns the fifth element of the list as its second argument.
E.g.
|?- fifth([1,2,3,4,5,6,7], X).
X = 5
yes
Recall that every list (the empty list) has both a head and
a tail. Use this fact, and the headtail notation to
define a predicate is_list/1 that returns true if its argument
is a list (including the empty list) and false otherwise.
![\begin{solution}\texttt{is\_list/1}
Two clauses, one for the empty list and o...
...begin{verbatim}
is_list([ ]).
is_list([_\vert _]).\end{verbatim}
\end{solution}](img17.png)
Define a predicate cons/3, which takes a list as its second
argument, anything as its first argument and returns as its third
argument a new list in with the first argument as head and the
second argument as tail.
![\begin{solution}\texttt{cons/3}
A version with explicit unification:
\begin{ve...
...s(First, List, [First\vert List]):-
is_list(List).\end{verbatim}
\end{solution}](img18.png)
Steve Harlow
2001-11-26