Function: expand-file-name

Convert filename NAME to absolute, and canonicalize it.
Second arg DEFAULT-DIRECTORY is directory to start with if NAME is relative
(does not start with slash or tilde); if DEFAULT-DIRECTORY is nil or missing,
the current buffer's value of `default-directory' is used.
NAME should be a string that is a valid file name for the underlying
File name components that are `.' are removed, and
so are file name components followed by `..', along with the `..' itself;
note that these simplifications are done without checking the resulting
file names in the file system.
Multiple consecutive slashes are collapsed into a single slash,
except at the beginning of the file name when they are significant (e.g.,
UNC file names on MS-Windows.)
An initial `~/' expands to your home directory.
An initial `~USER/' expands to USER's home directory.
See also the function `substitute-in-file-name'.

For technical reasons, this function can return correct but
non-intuitive results for the root directory; for instance,
(expand-file-name ".." "/") returns "/..". For this reason, use
(directory-file-name (file-name-directory dirname)) to traverse a
filesystem tree, not (expand-file-name ".." dirname). (fn NAME &optional DEFAULT-DIRECTORY)