Set operations

They try to be set-like, supporting most of the operations that the set object does.

Actually, dunno how much of this I'm missing. Raise a ticket if there are gaps.

Symbol	Name	Code
A ⋂ B	intersection	A & B
A ⋃ B	union	A | B
A ⊆ B	subset	A <= B
A ⊂ B	proper subset	A < B
A ⊇ B	superset	A >= B
A ⊃ B	proper superset	A > B
A = B	equality	A == B
A'	complement	~A
A \ B	relative complement	A - B
A ∆ B	symmetric difference	(A | B) - (A & B)
a ∈ A	membership	a in A
|A|	cardinality	len(A)
Ø	empty set	Ranges("")
ℕ0	natural numbers	Ranges(":")