Warning messages
----------------
Setting the environment variable `POSIXLY_CORRECT' turns off the
following warning and error messages, for strict compliance with
POSIX.2. Otherwise, the following diagnostics are issued:
1. When the `--delete' option is given but `--squeeze-repeats' is
not, and SET2 is given, GNU `tr' by default prints a usage message
and exits, because SET2 would not be used. The POSIX
specification says that SET2 must be ignored in this case.
Silently ignoring arguments is a bad idea.
2. When an ambiguous octal escape is given. For example, `\400' is
actually `\40' followed by the digit `0', because the value 400
octal does not fit into a single byte.
GNU `tr' does not provide complete BSD or System V compatibility.
For example, it is impossible to disable interpretation of the POSIX
constructs `[:alpha:]', `[=c=]', and `[c*10]'. Also, GNU `tr' does not
delete zero bytes automatically, unlike traditional Unix versions,
which provide no way to preserve zero bytes.