---
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" "str"
        str_content := rules "str_content" "str"
          str_content := rules 
          str := lexemes "STR"
            STR "a" (1, 2)
        str := lexemes "STR"
          STR "b" (1, 3)
      str := lexemes "STR"
        STR "c" (1, 4)
    string_end := lexemes "STRING_END"
      STRING_END "'" (1, 5)