+*
+* Since 2005.01.06, NumberlikeArray uses `NULL' rather
+* than a real array if one of zero length is needed.
+* These constructors implicitly call NumberlikeArray's
+* default constructor, which sets `blk = NULL, cap = len = 0'.
+* So if the input number is zero, they can just return.
+* See remarks in `NumberlikeArray.hh'.