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The most common examples arise with binary operators.  Methods may be
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The most common examples arise with binary operators.  Methods may be
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defined for individual operators, for special groups of operators such as
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defined for individual operators, for special groups of operators such as
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\code{\link{Arith}} or for group \code{\link{Ops}}.
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\code{\link{Arith}} or for group \code{\link{Ops}}.
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}
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}
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\section{Exporting Methods}{
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If a package defines methods for generic functions, those methods
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should be exported if any of the classes involved are exported; in
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other words, if someone using the package might expect these methods
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to be called.
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Methods are exported by including an \code{exportMethods()} directive
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in the \code{NAMESPACE} file for the package, with the arguments to
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the directive being the names of the generic functions for which
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methods have been defined.
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Exporting methods is always desirable in the sense of declaring what
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you want to happen, in that you do expect users to find such methods.
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It can be essential in the case that the method was defined for a
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function that is not originally a generic function in its own package
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(for example, \code{plot()} in the \code{graphics} package).  In this
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case it may be that the version of the function in the \R session is
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not generic, and your methods will not be called.
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Exporting methods for a function also exports the generic version of
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the function.
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Keep in mind that this does \emph{not} conflict with the function as
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it was originally defined in another package; on the contrary, it's
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designed to ensure that the function in the \R session dispatches
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methods correctly for your classes and continues to behave as expected
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when no specific methods apply.  See \link{Methods_Details} for the actual mechanism.
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}
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\section{Details}{
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\section{Details}{
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The call to \code{setMethod} stores the supplied method definition  in
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The call to \code{setMethod} stores the supplied method definition  in
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the metadata table for this generic function in the environment,
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the metadata table for this generic function in the environment,
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typically the global environment or the namespace of a package.
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typically the global environment or the namespace of a package.
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In the case of a package, the table object becomes part of the namespace or environment of the
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In the case of a package, the table object becomes part of the namespace or environment of the