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