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unix v7是最后一个广泛发布的研究型UNIX版本

.SH
Appendix D: Old Features Supported but not Encouraged
.PP
This Appendix mentions synonyms and features which are supported for historical
continuity, but, for various reasons, are not encouraged.
.IP 1.
Literals may also be delimited by double quotes ``"''.
.IP 2.
Literals may be more than one character long.
If all the characters are alphabetic, numeric, or \_, the type number of the literal is defined,
just as if the literal did not have the quotes around it.
Otherwise, it is difficult to find the value for such literals.
.IP
The use of multi-character literals is likely to mislead those unfamiliar with
Yacc, since it suggests that Yacc is doing a job which must be actually done by the lexical analyzer.
.IP 3.
Most places where % is legal, backslash ``\e'' may be used.
In particular, \e\e is the same as %%, \eleft the same as %left, etc.
.IP 4.
There are a number of other synonyms:
.DS
%< is the same as %left
%> is the same as %right
%binary and %2 are the same as %nonassoc
%0 and %term are the same as %token
%= is the same as %prec
.DE
.IP 5.
Actions may also have the form
.DS
={ . . . }
.DE
and the curly braces can be dropped if the action is a
single C statement.
.IP 6.
C code between %{ and %} used to be permitted at the
head of the rules section, as well as in the
declaration section.