static boolean prefixMatches(Tree target, Tree pattern) {
if (pattern.isEmpty()) return true;
if (target.isEmpty() || target.value() != pattern.value()) return false;
return prefixMatches(target.left(), pattern.left())
&& prefixMatches(target.right(), pattern.right());
}
/**
* Search the tree for the first instance of `pattern`, giving preference to the left child.
*
* <p>`pattern` is a tree, representing a pattern of nodes. `NilNode`s should be considered
* wildcards.
*
* <p>Example patterns ---------------- `Tree()` Matches any tree.
*
* <p>`Tree(3)` Matches any tree where the root node has a value of `3`.
*
* <p>`Tree(3, Tree(2), Tree())` Matches any tree where the root node has a value of `3`, and a
* left sub-tree with a root value of `2`, and any (or no) right sub-tree.
*/
static Optional<Tree> find(Tree target, Tree pattern) {
if (pattern.isEmpty()) return Optional.of(target);
if (target.isEmpty()) return Optional.empty();
if (prefixMatches(target, pattern)) return Optional.of(target);
Optional<Tree> leftMatch = find(target.left(), pattern);
if (leftMatch.isPresent()) return leftMatch;
return find(target.right(), pattern);
}