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  While \code{f} can correspond to methods defined on several packages
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  While \code{f} can correspond to methods defined on several packages
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  or environments, the underlying model is that these together make up
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  or environments, the underlying model is that these together make up
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  the definition for a single generic function.  When R proceeds to
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  the definition for a single generic function.  When R proceeds to
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  select and evaluate methods for \code{f}, the methods on the current
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  select and evaluate methods for \code{f}, the methods on the current
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  search list are merged to form a single methods list.  When \code{f}
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  search list are merged to form a single generic function and
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  associated methods list.  When \code{f}
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  is called and a method is \dQuote{dispatched}, the evaluator matches the
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  is called and a method is \dQuote{dispatched}, the evaluator matches the
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  classes of the actual arguments to the signatures of the available
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  classes of the actual arguments to the signatures of the available
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  methods.  When a match is found, the body of the corresponding
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  methods.  When a match is found, the body of the corresponding
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  method is evaluated (in R, the body is evaluated in the lexical
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  method is evaluated, but without rematching the arguments to
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  context of the method), but without rematching the arguments to
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  \code{f}.   Aside from not rematching the arguments, the computation
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  proceeds as if the call had been to the method.  In particular, the
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  \code{f}.
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  lexical scope of the method is used.
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  Method definitions can
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  have default expressions for arguments.  If those arguments are then
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  missing in the call to the generic function, the default expression
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  in the method is used.  If the method definition has no default for
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  the argument, then the expression (if any) supplied in the definition of the
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  generic function itself is used.  But note that this expression will
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  be evaluated in the environment defined by the method.
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  It is possible, however, to have some differences between the
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  It is possible to have some differences between the
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  formal arguments to a method supplied to \code{setMethod} and those
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  formal arguments to a method supplied to \code{setMethod} and those
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  of the generic. Roughly, if the generic has \dots as one of its
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  of the generic. Roughly, if the generic has \dots as one of its
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  arguments, then the method may have extra formal arguments, which
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  arguments, then the method may have extra formal arguments, which
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  will be matched from the arguments matching \dots in the call to
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  will be matched from the arguments matching \dots in the call to
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  \code{f}.  (What actually happens is that a local function is
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  \code{f}.  (What actually happens is that a local function is