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  the documentation of \code{\link{setOldClass}}.
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  the documentation of \code{\link{setOldClass}}.
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}
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}
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\section{Contained S3 Objects}{
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\section{Contained S3 Objects}{
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  Registering an S3 class defines an S4 class.  Objects from this
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  class are essentially identical in content to an object from the S3
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  class, except for two differences.  The value returned by
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  \code{\link{class}()} will always be a single string for the S4
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  object, and \code{\link{isS4}()} will return \code{TRUE} or
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  \code{FALSE} in the two cases.  See the example below.  It is barely
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  possible that some S3 code will not work with the S4 object; if so,
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  use \code{as(x, "S3")}.
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  Objects from a class that extends an S3 class will have some basic type and
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  Objects from a class that extends an S3 class will have some basic type and
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  possibly some attributes.  For an S3 class that has an equivalent S4
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  possibly some attributes.  For an S3 class that has an equivalent S4
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  definition (e.g., \code{"data.frame"}), an extending S4 class will
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  definition (e.g., \code{"data.frame"}), an extending S4 class will
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  have a data part and slots.  For other S3 classes (e.g., \code{"lm"}) an
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  have a data part and slots.  For other S3 classes (e.g., \code{"lm"}) an
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  object from the extending S4 class will be some sort of basic type,
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  object from the extending S4 class will be some sort of basic type,