Merge visitor: traverses the tree collecting per-node values into
a single result via a user-supplied merge function. Leaf nodes
(or nodes outside keys) yield leafValue(node); every other
visited node's value is merge(node, [<visited children's values>]).
Merge visitor: traverses the tree collecting per-node values into a single result via a user-supplied
mergefunction. Leaf nodes (or nodes outsidekeys) yieldleafValue(node); every other visited node's value ismerge(node, [<visited children's values>]).