match.fun is not intended to be used at the top level since it
will perform matching in the parent of the caller.
FUN is a function, it is returned. If it is a symbol (for
example, enclosed in backquotes) or a
character vector of length one, it will be looked up using
in the environment of the parent of the caller. If it is of any other
mode, it is attempted first to get the argument to the caller as a
substitute twice), and if that fails, an error is
descend = TRUE,
match.fun will look past non-function
objects with the given name; otherwise if
FUN points to a
non-function object then an error is generated.
This is used in base functions such as