Symbol names can contain letters, underscores, periods, dashes, and (not
at the beginning) digits. Dashes in symbol names are a GNU
-extension, incompatible with POSIX Yacc. Periods and dashes make symbol
-names less convenient to use with named references, which require brackets
-around such names (@pxref{Named References}). Terminal symbols
+extension, incompatible with POSIX Yacc. Terminal symbols
that contain periods or dashes make little sense: since they are not
valid symbols (in most programming languages) they are not exported as
token names.