CRAN policies

The 'No visible binding for global variable" is a good example.  This
found some bugs in my 'survey' package, which I removed. There is
still one note of this type, which arises when I have to handle two
different versions of the hexbin package with different internal
structures.  The note is a false positive because the use is guarded
by an if(), but  CMD check can't tell this.   So, it's a good idea to
remove all Notes that can be removed without introducing other code
problems, which is nearly all of them, but occasionally there may be a
good reason for code that produces a Note.

The survival package has a similar special case: the routines for 
expected population survival are set up to accept multiple types of date 
format so have lines like
     if (class(x) == 'chron') { y <- as.numeric(x - chron("01/01/1960")}
This leaves me with two extraneous "no visible binding" messages.  There 
used to be half a dozen but I've tried to remove as many as possible, 
for all the good reasons already articulated by the maintainers.

It still remains that 99/100 of the "no visible binding" messages I've 
seen over the years were misspelled variable names, and the message is a 
very welcome check.

Terry Therneau