// of bytes in the combined first two elements, error).
// The complete result is equal to
// `bytes.Join(append(fullBuffers, finalFragment), nil)`, which has a
// length of `totalLen`. The result is structured in this way to allow callers
// to minimize allocations and copies.
func (b *Reader) collectFragments(delim byte) (fullBuffers [][]byte, finalFragment []byte, totalLen int, err error) {
	var frag []byte

For performance reasons,
implementations may instead treat larger operations
as a set of individual machine-word-sized operations
in an unspecified order.
This means that races on multiword data structures
can lead to inconsistent values not corresponding to a single write.
When the values depend on the consistency
of internal (pointer, length) or (pointer, type) pairs,
as can be the case for interface values, maps,

[TestFS] now returns a structured error that can be unwrapped
(via method `Unwrap() []error`). This allows inspecting errors