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+Returns a string containing the characters in +self+, with cases reversed:
+
+- Each uppercase character is downcased.
+- Each lowercase character is upcased.
+
+Examples:
+
+  'Hello'.swapcase        # => "hELLO"
+  'Straße'.swapcase       # => "sTRASSE"
+  'RubyGems.org'.swapcase # => "rUBYgEMS.ORG"
+
+The sizes of +self+ and the upcased result may differ:
+
+  s = 'Straße'
+  s.size          # => 6
+  s.swapcase      # => "sTRASSE"
+  s.swapcase.size # => 7
+
+Some characters (and some character sets) do not have upcase and downcase versions;
+see {Case Mapping}[rdoc-ref:case_mapping.rdoc]:
+
+  s = '1, 2, 3, ...'
+  s.swapcase == s # => true
+  s = 'こんにちは'
+  s.swapcase == s # => true
+
+The casing is affected by the given +mapping+,
+which may be +:ascii+, +:fold+, or +:turkic+;
+see {Case Mappings}[rdoc-ref:case_mapping.rdoc@Case+Mappings].
+
+Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].