S4formals gives the formal arguments of the requested method.
If fun is not of class methodDefinition, it calls
getMethods, passing on all arguments.
Typically, fun is the name of as generic function and the
second argument is the signature of the method as a character
vector. Alternatively, fun may be the method itself (e.g. obtained
previously from getMethod) and in that case the \dots
arguments are ignored. See getMethod for full details
and other acceptable arguments.