Function replaceFolder

Converts a fold φ into one that replaces tree values: every tree node will be replaced with the intermediate value of the fold at the node.

For example, to convert the descendantCount fold, that folds a tree into a tree total descendant count to one where each node value is the node total descendant count:

const replaced = pipe(
  tree(42, [leaf(43)]),
  self, treeCata(replaceFolder(folds.descendantCount))),
)
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