# File ext/tools/preproc.rb, line 41
def prelude(f, out)
  while line = f.gets
    case line
    when %r</\*%%%\*/>
      out << '/*' << $/
    when %r</\*%>
      out << '*/' << $/
    when %r<%\*/>
      out << $/
    when /\A%%/
      out << '%%' << $/
      return
    when /\A%token/
      out << line.sub(/<\w+>/, '<val>')
    when /\A%type/
      out << line.sub(/<\w+>/, '<val>')
    else
      out << line
    end
  end
end