def modify(key, value, tree)
      # if already set, just change value
      return false unless tree[key]

      tree[key] = value
      true
    end
def modify(key, value, tree)
      # if already set, just change value
      return false unless tree[key]

      tree[key] = value
      true
    end
Modify an **existing** entry and return `true`,
 or do nothing and return `false`.
 @return [Boolean]