Designing S3 generics for extensibility

[hadley]

As long as the generic contains ..., R CMD check does not compare the method and generic args. See https://github.com/hadley/s3method for sample package that verifies that.

And R CMD check code (via @jimhester): https://github.com/wch/r-source/blob/7345bdb64fc8f6e30dff7077372137bd0e43fb27/src/library/tools/R/QC.R#L2430-L2434

[DavisVaughan]

This is not entirely true, see https://github.com/DavisVaughan/methodtest for a counterpoint where ... is in the generic and we still get a warning