みっつめの例が変数名が間違ってました。下に訂正したものを載せます。

# # d + ""は新たな文字列オブジェクトを生成するので参照元は影響されない。  
# d = "hiroe"  
# e = d + ""  
# e.concat("shin")  
# p d #=> "hiroe"  
# p e #=> "hiroeshin"

修正しておきました。

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