---
Γ := rules "string"
  string := rules "string_start" "str_content" "string_end"
    string_start := lexemes "STRING_START"
      STRING_START "'" (1, 1)
    str_content := rules "str_content" "str"
      str_content := rules "str_content" "var"
        str_content := rules "str_content" "str"
          str_content := rules "str_content" "var"
            str_content := rules "str_content" "str"
              str_content := rules 
              str := lexemes "STR"
                STR "a" (1, 2)
            var := lexemes "VAR"
              VAR "b" (1, 6)
          str := lexemes "STR"
            STR "c" (1, 9)
        var := lexemes "VAR"
          VAR "d" (1, 13)
      str := lexemes "STR"
        STR "e" (1, 16)
    string_end := lexemes "STRING_END"
      STRING_END "'" (1, 17)