Trigraph sequences
Trigraph sequences are three-character
sequences that are replaced by a corresponding
single character in Translation Phase 1, as follows:
??= # ??( [ ??< {
??/ \ ??) ] ??> }
??' ^ ??! | ??- ~
No other such sequences are recognized.
The trigraph sequences provide a way to
specify characters that are missing on some
terminals, but that the C language uses.