Values of this type denote a (sorted) set of zettel identifier.
A set is different to a list, as no duplicate values are allowed.
Allowed values
Must be at least one sequence of 14 digits (0–9), separated by space characters.
Query comparison
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
Sorting is done by comparing the String values.