%!demo
%! ## As a `calendarDuration` *method*, `calmonths` inverts the builder: it returns
%! ## the whole span expressed as a number of months (years and months rolled
%! ## together).  Compare `split`, which instead decomposes the span into parts.
%!
%! d = calendarDuration (2, 3, 40)
%! calmonths (d)
