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Subject: x-deltat.y is not compatible with bison 3
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bison 3 removes support for YYLEX_PARAM and YYPARSE_PARAM, which we use 
in x-deltat.y.  We are supposed to use %lex-param and %parse-param 
instead.

bison also warns about %pure_parser, saying we should use %pure-parser 
instead.  %pure-parser is itself documented as deprecated; the modern 
idiom actually seems to be "%define api.pure".