Values of this type denote a (sorted) set of zettel identifier.
A set is different to a list, as no duplicate values are allowed.
Must be at least one sequence of 14 digits (“0”–“9”), separated by space characters.
A value matches an identifier set value, if the value matches any of the identifier set values.
For example, “000010060325” is a prefix “00001006032000 00001006032500”.
Sorting is done by comparing the String values.