[file tail ~fred/wilma] will return "wilma", but [file
tail ~fred] will error, if there is no ~fred (that is,
if the user fred does not exist.)
It seems that when the terminal element of a file
path/name is ~, it is expanded.
I think that there's no good reason for [file tail] to
have an existential interpretation, but that
normalization of the path with a ~-element forces the
existence to be tested implicitly and unintentionally.