- %nonassoc should probably produce an error when %nonassoc operators are used
  repeatedly. 

block.h: skip should expect std::string const &, see also skip-calls in
e.g. lexer.  Same for Block constructor

--print dropped, use --display-tokens instead without argument

Parser::print() calls Parser::print__, implemented in lex.cc, but is commented
out by default. Uncomment or control by option if tokens should be displayed.
