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Standard type for blit functions, and reusable code for validating blit arguments.
type('src, 'dst) blit =
src:'src->src_pos:int ->dst:'dst->dst_pos:int ->len:int ->
unit
If blit : (src, dst) blit, then blit ~src ~src_pos ~len ~dst ~dst_pos blits len values from src starting at position src_pos to dst at position dst_pos. Furthermore, blit raises if src_pos, len, and dst_pos don't specify valid slices of src and dst.
type('src, 'dst) blito =
src:'src->?src_pos:int ->?src_len:int ->dst:'dst->?dst_pos:int ->unit ->
unit
blito is like blit, except that the src_pos, src_len, and dst_pos are optional (hence the "o" in "blito"). Also, we use src_len rather than len as a reminder that if src_len isn't supplied, then the default is to take the slice running from src_pos to the end of src.
type('src, 'dst) sub = 'src->pos:int ->len:int ->'dst
If sub : (src, dst) sub, then sub ~src ~pos ~len returns a sequence of type dst containing len characters of src starting at pos.
subo is like sub, except pos and len are optional.
Users of modules matching the blit signatures S, S1, and S1_distinct only need to understand the code above. The code below is only for those that need to implement modules that match those signatures.