Many R-internal functions are generic and allow methods to be written for.
Details
The following primitive and internal functions are generic, i.e., you can write methods for them: [, [[, $, [<-, [[<-, $<-, length, length<-, dimnames, dimnames<-, dim, dim<-, names, names<-, levels<-, c, unlist, cbind, rbind, as.character, as.complex, as.double, as.integer, as.logical, as.raw, as.vector, is.array, is.matrix, is.na, is.nan, is.numeric, rep, seq.int (which dispatches methods for "seq") and xtfrm In addition, is.name is a synonym for is.symbol and dispatches methods for the latter. Similarly, as.numeric is a synonym for as.double and dispatches methods for the latter, i.e., S3 methods are for as.double, whereas S4 methods are to be written for as.numeric. Note that all of the group generic functions are also internal/primitive and allow methods to be written for them. .S3Pri