Determines if a file name refers to a directory
(vl-file-directory-p filename)
T, if filename is the name of a directory; nil if it is not.
_$ (vl-file-directory-p "sample")
T
_$ (vl-file-directory-p "yinyang")
nil
_$ (vl-file-directory-p "c:/My Documents")
T
_$ (vl-file-directory-p "c:/My Documents/visuallisp/yinyang.lsp")
nil