iterator before_begin() noexcept;
const_iterator before_begin() const noexcept;
const_iterator cbefore_begin() const noexcept;
Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by begin().
Effects: cbefore_begin() is equivalent to const_cast<forward_list const&>(*this).before_begin().
Remarks: before_begin() == end() shall equal false.