Values of this type denote a (sorted) set of tags.
A set is different to a list, as no duplicate values are allowed.
Every tag must must begin with the number sign character (
#, U+0023), followed by at least one printable character. Tags are separated by space characters.
All characters are mapped to their lower case values.
All comparisons are done case-sensitive, i.e.
#hell will not be the prefix of
Sorting is done by comparing the String values.