# File lib/hashery/castinghash.rb, line 95
  def rekey!(*args, &block)
    # for backward comptability (DEPRECATE?).
    block = args.pop.to_sym.to_proc if args.size == 1
    if args.empty?
      block = lambda{|k| k} unless block
      keys.each do |k|
        nk = block[k]
        self[nk] = delete(k) #if nk
      end
    else
      raise ArgumentError, "3 for 2" if block
      to, from = *args
      self[to] = delete(from) if has_key?(from)
    end
    self
  end