19.2.3 Merging Pathnames

Merging takes a pathname with unfilled components and supplies values for those components from a source of defaults.

If a component’s value is nil, that component is considered to be unfilled. If a component’s value is any non-nil object, including :unspecific, that component is considered to be filled.

Except as explicitly specified otherwise, for functions that manipulate or inquire about files in the file system, the pathname argument to such a function is merged with *default-pathname-defaults* before accessing the file system (as if by merge-pathnames).