Constructs a pathname from pathname
by filling in any unsupplied components with the corresponding values
from default-pathname and default-version.
Defaulting of pathname components
is done by filling in components taken from another pathname.
This is especially useful for cases such as
a program that has an input file and an output file.
Unspecified components of the output pathname will come from the input pathname,
except that the type should not default
to the type of the input pathname
but rather to the appropriate default type for output from the program;
for example, see the function compile-file-pathname.
If no version is supplied, default-version is used.
If default-version is nil, the version component will remain unchanged.
If pathname explicitly specifies a host and not a device, and
if the host component of default-pathname matches the host component
of pathname, then the device is taken from the default-pathname;
otherwise the device will be the default file device for that host. If
pathname does not specify a host, device, directory, name,
or type, each such component is copied from default-pathname.
If pathname does not specify a name, then the version, if not provided, will
come from default-pathname, just like the other components. If
pathname does specify a name, then the version is not affected
by default-pathname. If this process leaves the
version missing, the default-version is used.
If the host's file name syntax provides a way
to input a version without a name or type, the user can let the name
and type default but supply a version different from the one in default-pathname.
If pathname is a stream, pathname effectively
becomes (pathname pathname). merge-pathnames
can be used on either an open or a closed stream.
If pathname is a pathname
it represents the name used to open the file. This may be, but is
not required to be, the actual name of the file.
merge-pathnames recognizes a logical pathname namestring
when default-pathname is a logical pathname,
or when the namestring begins with
the name of a defined logical host followed by a colon.
In the first of these two cases,
the host portion of the logical pathname namestring
and its following colon are optional.
merge-pathnames returns a
logical pathname if and only if
its first argument is a logical pathname,
or its first argument is a logical pathname namestring with an explicit host,
or its first argument does not specify a host
and the default-pathname is a logical pathname.
Pathname merging treats a relative directory specially.
If (pathname-directory pathname) is a list whose
car is :relative, and
(pathname-directory default-pathname) is a list, then
the merged directory is the value of
(append (pathname-directory default-pathname)
(cdr ;remove :relative from the front
(pathname-directory pathname)))
except that if the resulting list contains a string or :wild
immediately followed by :back, both of them are removed. This removal of
redundant :back keywords is repeated as many times as possible.
If (pathname-directory default-pathname) is not a list or
(pathname-directory pathname) is not a list
whose car is :relative, the merged directory is
(or (pathname-directory pathname) (pathname-directory default-pathname))
merge-pathnames maps customary case in pathname
into customary case in the output pathname.
19 Filenames 
merged-pathname