PMS currently says: "A whitespace-separated list of versions of this
package (including revision, if specified) that are being replaced
(uninstalled or overwritten) as a result of this install."

Shouldn't the spec guarantee that it's the empty string (i.e., not
a string containing arbitrary whitespace) if the list is empty?
Otherwise simple binary tests like [[ -n ${REPLACING_VERSIONS} ]]
cannot be done.

For REPLACED_BY_VERSION, it is explicitly specified: "The single
version ..., or an empty string otherwise."

Ulrich