## mark a section (Learn more)

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"

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