The help page means exactly what it says, but the English is too subtle
and I have reworded it.
I have no idea why you are interested in pairlists (they are hardly used
at user-visible level these days). The point is that pairlist() is NULL
and so strictly not a pairlist at all (try typeof()
the help page says:
'is.list' returns 'TRUE' iff its argument is a 'list' _or_ a
'pairlist' of 'length' 0, whereas 'is.pairlist' only returns
'TRUE' in the latter case.
does the latter case mean a 'pairlist' of 'length' 0?
but
is.pairlist(pairlist())
[1] TRUE
length(pairlist())
