def index data
      current = @first
      i = 0
      while !current.nil?
        return i if current.data == data
        current = current.next
        i += 1
      end
      nil
    end
def index data
      current = @first
      i = 0
      while !current.nil?
        return i if current.data == data
        current = current.next
        i += 1
      end
      nil
    end
Returns the first index equal to +data+ (using == comparison).
 Counts from the beginning of the list.