IdentifierSet Key Type
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 (
9), separated by space characters.
A value matches an identifier set value, if the value matches any of the identifier set values.
000010060325 is a prefix
Sorting is done by comparing the String values.