Mark Function as Non-Printing
DESCRIPTION:
Insures that an object will not be printed.
USAGE: 
invisible(x=NULL)
OPTIONAL ARGUMENTS:
x:
any S-PLUS object.
VALUE:
SIDE EFFECTS:
the function invisible reaches back 2 frames and sets a special flag, to FALSE. This has the effect of preventing automatic printing if the function calling invisible was called from the top level since .Auto.print in frame 1 is set to FALSE.
DETAILS:
Note that, if a function wants to return an invisible value if the function it calls does so, it can check .Auto.print, as in the checking example below.
SEE ALSO:
amp;.Program , print .
EXAMPLES: 
function(...) {
amp;...
plot(x,y)
invisible(list(x,y))
} # return a value invisibly


checking <- function(fun,...) {
        .Auto.print <- TRUE
        val <- fun(...) # may change .Auto.print
        if(.Auto.print) val
        else invisible(val)
}