# expect_s[34]_class can check not S3/S4

    factor() is an S3 object

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    A() is an S4 object

# test_s4_class respects class hierarchy

    C() inherits from 'C'/'A'/'B'/'list'/'vector' not 'D'.

# test_s3_class respects class hierarchy

    `x` inherits from 'a'/'b' not 'c'.

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    `x` inherits from 'a'/'b' not 'c'/'d'.

