With Bison being shown the door in 4.0-CURRENT, we need the Bison port back.

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+Bison is a tool used to write parsers, such as the parser for GNU cc.
+It is similar to Yacc, which is included in the base FreeBSD system.
+
+The main difference between Bison and Yacc that I know of is that
+Bison supports the @N construction, which gives you access to
+the starting and ending line number and character number associated
+with any of the symbols in the current rule.
+
+Also, Bison supports the command `%expect N' which says not to mention
+the conflicts if there are N shift/reduce conflicts and no reduce/reduce
+conflicts.
+
+The differences in the algorithms stem mainly from the horrible
+kludges that Johnson had to perpetrate to make Yacc fit in a PDP-11.
+
+Also, Bison uses a faster but less space-efficient encoding for the
+parse tables (see Corbett's PhD thesis from Berkeley, "Static
+Semantics in Compiler Error Recovery", June 1985, Report No. UCB/CSD
+85/251), and more modern technique for generating the lookahead sets.
+(See "Efficient Construction of LALR(1) Lookahead Sets" by F. DeRemer
+and A. Pennello, in ACM TOPLS Vol 4 No 4, October 1982.  Their
+technique is the standard one now.)