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