---
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)