Unicode Sets in 'Unicode Regular Expressions'

[Richard Wordingham]

UTS#18 'Unicode Regular Expressions' Version 17 Requirement RL1.3
'Subtraction and Intersection' talks of Unicode sets.  What is the
relevant definition of a 'Unicode set'? Is it a finite set of non-empty
strings?  Other possibilities that occur to me, depending on context,
include sets of codepoints and sets of indecomposable codepoints.

Richard.