- %nonassoc should probably produce an error when %nonassoc operators are used
  repeatedly. 
- make error recovery configurable in the sense that a series of tokens must
  have been successfully processed before error messages are generated again
  (see documentation/manual/error/intro.yo)

- Special character @ in skeletons must be changed into something
  else. Suggestion: % or \@
    Problem with @ is that it's used in e-mail addresses (e.g., in my e-mail
    address which is used in share/bisonc++.cc
- Repair occurrence of stack underflow 