It may be interesting for file readers to return both decoded contents
and also diagonstics, for example when doing error recovery while
transcoding the source buffer.
The Read primitives currently does not allow this: Contents is
considered uninitialized when Diagnostics is not empty. Always expect
Contents to be initialized instead to support this case.
TN: VA17-001
On Windows, the default encoding (not UTF-8) may not allow non-ASCII
test outputs. Use Python's "ascii" builtin where needed to ensure pure
ASCII test outputs.
TN: U720-016
