[Rd] best reference on generics

[Spencer Graves]

Hello, All:

      What would you say is the best succinct reference to cite on use
of generic functions, especially S3 generics?

      I want to add an appropriate citation on this to a forthcoming
book in the Springer "useR!" series (discussing the use of the 'fda'
package, which uses the S3 standard).

      Thanks,
      Spencer Graves