Initial data commit

Task/Set/Sidef/set-2.sidef

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+var x = MySet(1, 2, 3)
+5..7 -> each { |i| x += i }
+
+var y = MySet(1, 2, 4, x)
+
+say "set x is: #{x}"
+say "set y is: #{y}"
+
+[1,2,3,4,x].each { |elem|
+    say ("#{elem} is ", elem ∈ y ? '' : 'not', " in y")
+}
+
+var (w, z)
+say ("union: ", x ∪ y)
+say ("intersect: ", x ∩ y)
+say ("z = x ∖ y = ", z = (x ∖ y) )
+say ("y is ", x ⊆ y ? "" : "not ", "a subset of x")
+say ("z is ", x ⊆ z ? "" : "not ", "a subset of x")
+say ("z = (x ∪ y) ∖ (x ∩ y) = ", z = ((x ∪ y) ∖ (x ∩ y)))
+say ("w = x ^ y = ", w = (x ^ y))
+say ("w is ", w ≡ z ? "" : "not ", "equal to z")
+say ("w is ", w ≡ x ? "" : "not ", "equal to x")