   INTERVAL(N,N1,N2,N3,...)
Returns 0 if N < N1, 1 if N < N2
and so on or -1 if N is NULL. All arguments are treated
as integers.  It is required that N1 < N2 < N3 <
... < Nn for this function to work correctly. This is because
a binary search is used (very fast).
