Optional Readonlydefaultswhat a parameter R gives a default stands for, so paste(a, b) folds like paste(a, b, sep = ' ')
Readonlyfoldthe fold over the supplied arguments, in declaration order
Optional Readonlyignoredparameters a call may supply that change nothing for the single strings we fold, like paste's collapse
Readonlyparamsthe parameter names, in the order R declares them; ... collects the arguments naming no other parameter
Readonlypkgthe package declaring it, so another package's function of that name does not fold
One entry of the StringFns registry: the parameters R declares, in order, and how to fold them.
foldreceives them in that order, so a parameter R gives a default is an optional parameter offold, and a...parameter arrives as the array of everything it collected. Whateverfoldcannot answer it returnsundefinedfor, which keeps the callTop.