It just needs to
return the wild-card substitution that
makes them match:
(substitutions-in-to-match
'(who acted in ...)
'(who acted in boyz n the hood)) ⇒ ((boyz n the hood))
Q: Why not just (boyz n the hood)?
A:
Because the query system will later allow
more than one wild-card in the pattern, for
example,
(what movies were made between _ and _ )
"_" is another kind of wild-card introduced next.
substitutions-in-to-match is left as an exercise (7.26).