expect_true(unique_characters('abc'))
expect_true(unique_characters(' '))
expect_true(unique_characters(''))
expect_true(all(unique_characters(c('abc', ' ', ''))))
expect_false(unique_characters('aa'))
expect_false(unique_characters('a a'))
expect_false(unique_characters('abba'))
expect_false(all(unique_characters(c('aa', 'a a', 'abba'))))
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