reductions.rb

module Enumerable
  def reductions(sym = nil, &block)
    raise LocalJumpError, "no block given" unless sym || block
    
    if sym
      block = sym.to_proc
    end
    
    arr = to_a
    
    arr[1..-1].reduce([arr.first]) do |acc, o|
      acc << block.call(acc.last, o)
    end
  end
end