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irb(main):001:0> s = "ab"
=> "ab"
irb(main):002:0> s + "c"
=> "abc"
irb(main):003:0> s
=> "ab"
irb(main):004:0> t = "ab"
=> "ab"
irb(main):005:0> t << "c"
=> "abc"
irb(main):006:0> t
=> "abc"
irb(main):007:0> u = "ab"
=> "ab"
irb(main):008:0> u += "c"
=> "abc"
irb(main):009:0> u
=> "abc"