For the functions in Figure~19–2,
a value of :common
for the :case argument
that these functions should receive
and yield strings in component values according to the following conventions:
*All uppercase means to use a file system’s customary case.
*All lowercase means to use the opposite of the customary case.
*Mixed case represents itself.
Note that these conventions have been chosen in such a way that translation
from :local to :common and back to :local is information-preserving.