R < 3.5.0 will error when loading help files due to lack of isFALSE()

[nathaneastwood]

isFALSE() was defined in R 3.5.0 and so if you are using a version of R before this version, you will see an error when trying to load a help file

?rnorm
# Error in isFALSE(viewer) : could not find function "isFALSE"

You could define your own version of isFALSE() easily enough with

isFALSE <- function(x) is.logical(x) && length(x) == 1L && !is.na(x) && !x

for those who are not yet up to date with their R version.