## all as.*() and is.*() in 'base' and 'defaultPackages'
pkg <- c("base", "datasets", "grDevices", "graphics",
         "methods", "stats", "utils")
getExports <- function(s) names(as.environment(paste0("package:", s)))
nms <- sort(unlist(lapply(pkg, getExports)))
as. <- nms[startsWith(nms, "as.")]
is. <- nms[startsWith(nms, "is.")]

## excluding methods and assignments
as.. <- c("as.data.frame", grep("([.].+[.]|<-)", as., value=TRUE, invert=TRUE))
is.. <- c("is.data.frame", grep("([.].+[.]|<-)", is., value=TRUE, invert=TRUE))

cl.as <- sub("^as[.]", "", as..)
cl.is <- sub("^is[.]", "", is..)

callsInherits <- function(x)
    is.call(b <- body(match.fun(x))) && b[[1L]] == "inherits"

## missing as.*()
setdiff(cl.is, cl.as)
## missing is.*()
setdiff(cl.as, cl.is)
## is.*() just calling inherits()
cl.is[vapply(is.., callsInherits, NA)]
