🚧✨ Add `SequenceSet#xor!"

This is simply a mutating version of `^` or `xor`.

TODO:
* rdoc on method
* rdoc in class "What's Here"

Author: nevans

lib/net/imap/sequence_set.rb

@@ -739,7 +739,7 @@ def &(other) remain_frozen dup.intersect! other end
       # * <tt>(lhs | rhs) - (lhs & rhs)</tt>
       # * <tt>(lhs - rhs) | (rhs - lhs)</tt>
       # * <tt>(lhs ^ other) ^ (other ^ rhs)</tt>
-      def ^(other) remain_frozen (dup | other).subtract(self & other) end
+      def ^(other) remain_frozen dup.xor! other end
       alias xor :^
 
       # :call-seq:
@@ -1451,6 +1451,15 @@ def intersect!(other) # :nodoc:
         subtract SequenceSet.new(other).complement!
       end
 
+      # TODO: document and directly test this
+      def xor!(other) # :nodoc:
+        modifying!
+        copy  = dup
+        other = SequenceSet.new(other)
+        # subtract(other).merge(other.subtract(copy))      # (L - R) | (R - L)
+        merge(other).subtract(copy.intersect!(other))    # (L | R) - (L & R)
+      end
+
       # Returns a new SequenceSet with a normalized string representation.
       #
       # The returned set's #string is sorted and deduplicated.  Adjacent or

test/net/imap/test_sequence_set.rb

@@ -99,6 +99,9 @@ def compare_to_reference_set(nums, set, seqset)
       assert_equal xor, (lhs | rhs) - (lhs & rhs)
       assert_equal xor, (lhs ^ mid) ^ (mid ^ rhs)
       assert_equal xor, ~lhs ^ ~rhs
+      mutable = lhs.dup
+      assert_equal xor, mutable.xor!(rhs)
+      assert_equal xor, mutable
     end
   end