code: function treeSetMutable (address, tree, value) {
    if (address.length === 0) {
      return value
    } else {
      const [ rest, last ] = tail(address)
      const parent = treeGet(rest, tree)
      if (checkType(NODE, parent)) {
        parent.children[last] = value
      } else {
        parent[last] = value
      }
      return tree
    }
  }
docs: Set the value in a tree; mutable.
  @param {Array} address -
  @param {Object} tree -
  @param {*} value - The new value to set at address.
  @return (*) The tree.