The break points (NaN among them is dropped, as R does; fewer than 2 left are an error), or a
number of intervals (at least 2): then equal intervals over the range of x, widened by 1/1000 of the range at
each end, as R does. That range must be finite.
Interval numbers (1-based) of the values, or NaN outside the breaks. Equivalent to R's
cut(..., labels = FALSE).