   def reduce(collection, acc, fun)
 
   @doc """
-  Check if a value exists within the collection.
+  Checks if a value exists within the collection.
 
   Membership should be tested with the match (`===`) operator.
   """

  def member?(collection, value)

  @doc """
  Retrieve the collection' size.
  """
  @spec count(t) :: non_neg_integer
  def count(collection)

  @type continuation :: (acc -> result)

  @doc """
  Reduces the collection into a value.

  Most of the operations in `Enum` are implemented in terms of reduce.
  This function should simply apply the given `reducer` function to each
  item in the collection and proceed as expected by the returned accumulator.

  As an example, here is the implementation of `reduce` for lists:
