There are various Make* functors that turn an unsafe_blit function into a blit function. The functors differ in whether the sequence type is monomorphic or polymorphic, and whether the src and dst types are distinct or are the same.
The blit functions make sure the slices are valid and then call unsafe_blit. They guarantee at a call unsafe_blit ~src ~src_pos ~dst ~dst_pos ~len that:
len > 0
&& src_pos >= 0
&& src_pos + len <= get_src_len src
&& dst_pos >= 0
&& dst_pos + len <= get_dst_len dst
The Make* functors also automatically create unit tests.