Relation between component values NIL and ->UNSPECIFIC (ANSI and GNU Common Lisp Document)

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#### 19.2.2.10 Relation between component values NIL and :UNSPECIFIC

If a pathname is converted to a namestring, the symbols nil and `:unspecific` cause the field to be treated as if it were empty. That is, both nil and `:unspecific` cause the component not to appear in the namestring.

However, when merging a pathname with a set of defaults, only a nil value for a component will be replaced with the default for that component, while a value of `:unspecific` will be left alone as if the field were “filled”; see the function merge-pathnames and Merging Pathnames.