#### 19.2.2.15 Restrictions on Examining a Pathname Directory Component

The directory might be a string, `:wild`, `:unspecific`, or nil.

The directory can be a list of strings and symbols.

The car of the list is one of the symbols `:absolute` or `:relative` , meaning:

`:absolute`

  A list whose car is the symbol `:absolute` represents a directory path starting from the root directory. The list `(:absolute)` represents the root directory. The list `(:absolute "foo" "bar" "baz")` represents the directory called `"/foo/bar/baz"` in Unix (except possibly for case).

`:relative`

  A list whose car is the symbol `:relative` represents a directory path starting from a default directory. The list `(:relative)` has the same meaning as nil and hence is not used. The list `(:relative "foo" "bar")` represents the directory named `"bar"` in the directory named `"foo"` in the default directory.

Each remaining element of the list is a string or a symbol.

Each string names a single level of directory structure. The strings should contain only the directory names themselves—no punctuation characters.

In place of a string, at any point in the list, symbols can occur to indicate special file notations. Figure 19–3 lists the symbols that have standard meanings. Implementations are permitted to add additional objects of any type that is disjoint from string if necessary to represent features of their file systems that cannot be represented with the standard strings and symbols.

Supplying any non-string, including any of the symbols listed below, to a file system for which it does not make sense signals an error of type file-error. For example, Unix does not support `:wild-inferiors` in most implementations.

      Symbol           Meaning
      :wild            Wildcard match of one level of directory structure
      :wild-inferiors  Wildcard match of any number of directory levels
      :up              Go upward in directory structure (semantic)
      :back            Go upward in directory structure (syntactic)

              Figure 19–3: Special Markers In Directory Component

The following notes apply to the previous figure:

Invalid Combinations

  Using `:absolute` or `:wild-inferiors` immediately followed by `:up` or `:back` signals an error of type file-error.

Syntactic vs Semantic

  “Syntactic” means that the action of `:back` depends only on the pathname and not on the contents of the file system.

  “Semantic” means that the action of `:up` depends on the contents of the file system; to resolve a pathname containing `:up` to a pathname whose directory component contains only `:absolute` and strings requires probing the file system.

  `:up` differs from `:back` only in file systems that support multiple names for directories, perhaps via symbolic links. For example, suppose that there is a directory `(:absolute "X" "Y" "Z")` linked to `(:absolute "A" "B" "C")` and there also exist directories `(:absolute "A" "B" "Q")` and `(:absolute "X" "Y" "Q")`. Then `(:absolute "X" "Y" "Z" :up "Q")` designates `(:absolute "A" "B" "Q")` while `(:absolute "X" "Y" "Z" :back "Q")` designates `(:absolute "X" "Y" "Q")`