lex-comment. =
   extends $(Lexer)

   level = 0

   other: .
      lex()

   term: $'[*][)]'
      if $(not $(eq $(level), 0))
         level = $(sub $(level), 1)
         lex()

   next: $'[(][*]'
      level = $(add $(level), 1)
      lex()

   eof: $"\'"
      eprintln(Unterminated comment)

lexer1. =
   extends $(Lexer)

   other: .
      eprintln(Illegal character: $0)
      lex()

   white: $'[[:space:]]+'
      lex()

   comment: $'[(][*]'
      lex-comment.lex-channel($(channel))
      lex()

   op: $'[-+*/()]'
      switch $0
      case +
         Token.unit($(loc), plus)
      case -
         Token.unit($(loc), minus)
      case *
         Token.unit($(loc), mul)
      case /
         Token.unit($(loc), div)
      case $'('
         Token.unit($(loc), lparen)
      case $')'
         Token.unit($(loc), rparen)

   number: $'[[:digit:]]+'
      Token.pair($(loc), exp, $(int $0))

   eof: $"\'"
      Token.unit($(loc), eof)

parser1. =
   extends $(Parser)

   #
   # Use the main lexer
   #
   lexer = $(lexer1)

   #
   # Precedences, in ascending order
   #
   left(plus minus)
   left(mul div)
   right(uminus)

   #
   # A program
   #
   start(prog)

   prog: exp eof
      value $1

   #
   # Simple arithmetic expressions
   #
   exp: minus exp :prec: uminus
      neg($2)

   exp: exp plus exp
      add($1, $3)

   exp: exp minus exp
      sub($1, $3)

   exp: exp mul exp
      mul($1, $3)

   exp: exp div exp
      div($1, $3)

   exp: lparen exp rparen
      value $2

println(Test1: should print 48)
println(Parsed $(parser1.parse-file prog, Test.input1))

lexer2. =
   extends $(lexer1)

   lsl: $"<<"
      Token.unit($(loc), lsl)

   lsr: $">>>"
      Token.unit($(loc), lsr)

   asr: $">>"
      Token.unit($(loc), asr)

parser2. =
   extends $(parser1)

   left(plus, lsl lsr asr)

   lexer = $(lexer2)

   exp: exp lsl exp
      lsl($1, $3)

   exp: exp lsr exp
      lsr($1, $3)

   exp: exp asr exp
      asr($1, $3)

println(Test2: should print 256)
println(Parsed $(parser2.parse-file prog, Test.input2))