---
string := string_start str_content string_end
  string_start := "STRING_START"
    STRING_START "`" (1, 1)
  str_content := str_content str
    str_content := str_content str
      str_content := str_content str
        str_content := 
        str := "STR"
          STR "a" (1, 2)
      str := "STR"
        STR "b" (1, 3)
    str := "STR"
      STR "c" (1, 4)
  string_end := "STRING_END"
    STRING_END "`" (1, 5)