For two reasons, one would like also to find the method or the
signature corresponding to the current, actual call to the function.
First, this is a useful prelude to doing some non-standard generic
computations, such as virtual method
Second, the correct method cannot always be determined by the classes
of the argument alone.
Suppose, for example, the actual argument x has class
"matrix", but the only methods defined correspond to
"character" and "numeric".
Then the method to be chosen could be either of these or the default,
depending on the class of the data contained in the matrix.
In the extension, selectMethod and selectMethodSignature are called with no arguments. They then call internal code in the evaluator to carry out method selection for the actual arguments to the call from which selectMethod or selectMethodSignature was called.