def ycomb
      proc { |f| f.call(f) }.call(proc { |f| yield proc { |*x| f.call(f).call(*x) } })
    end
def ycomb
      proc { |f| f.call(f) }.call(proc { |f| yield proc { |*x| f.call(f).call(*x) } })
    end
this is the Y-combinator, which allows anonymous recursive functions. for a simple example,
 to define a recursive function to return the length of an array:

    length = ycomb do |len|
      proc{|list| list == [] ? 0 : 1 + len.call(list[1..-1]) }
    end

 see https://secure.wikimedia.org/wikipedia/en/wiki/Fixed_point_combinator#Y_combinator
 and chapter 9 of the little schemer, available as the sample chapter at http://www.ccs.neu.edu/home/matthias/BTLS/