implify translation.  In particular, the message "memory exhausted"
  has replaced "parser stack overflow", as the old message was not
  always accurate for modern Bison-generated parsers.

** Destructors are now called when the parser aborts, for all symbols left
  behind on the stack.  Also, the start symbol is now destroyed after a
  successful parse.  In both cases, the behavior was formerly inconsistent.

** When generating verbose diagnostics, Bison-generated parsers no longer
  quote the literal strings associated with tokens.  For example, for
  a syntax error associated with '%token NUM "number"' they might
  print 'syntax error, unexpected number' instead of 'syntax error,
  unexpected "number"'.