function prod(*a)
   if len(a)==0
      return iter([[]])
   else
      return [x]+t for x in a[0] for t in prod(*a[1..])
   end
end

function permutations(a)
   a = list(a)
   return fn*||
      if len(a)<=1
         yield copy(a)
      else
         x = a[..0]
         for p in permutations(a[1..])
            for i in len(a)
               yield p[..i-1]+x+p[i..]
            end
         end
      end
   end
end

function combinations(a,k)
   a = list(a)
   n = len(a)
   return fn*||
      if k>n then return empty end
      indices = list(k)
      yield list(a[i] for i in indices)
      while true
         finish = true
         for i in k-1..0: -1
            if indices[i] != i+n-k
               finish = false
               break
            end
         end
         if finish then return empty end
         indices[i] += 1
         for j in i+1..k-1
            indices[j] = indices[j-1]+1
         end
         yield list(a[i] for i in indices)
      end
   end
end

function accumulate(a,f,g=null)
   if g is null
      i = iter(a)
      return fn*||
         y = i()
         yield y
         for x in i
            y = f(y,x)
            yield y
         end
      end
   else
      y = f
      f = g    
      return fn*||
         yield y
         for x in a
            y = f(y,x)
            yield y
         end
      end
   end
end

function chain(*argv)
   return fn*||
      for a in argv
         for x in a
            yield x
         end
      end
   end
end

function repeat(x,n=null)
   if n is null
      return fn*||
        while true
           yield x
        end
      end
   else
      return fn*||
        for i in n
           yield x
        end
      end
   end
end

Iterable.step = fn step|m|
   i = iter(self)
   n = m-1
   return fn*||
      while true
          yield i()
          for k in n do i() end
      end
   end
end