Add # needs

Author: Matthias Koeppe

src/sage/tests/book_stein_ent.py

@@ -33,7 +33,7 @@
 sage: n.is_prime()              # this is instant
 False
 sage: p = 2^32582657 - 1
-sage: p.ndigits()
+sage: p.ndigits()                                                                       # needs sage.rings.real_interval_field
 9808358
 sage: s = p.str(10) # this takes a long time
 sage: len(s)        # s is a very long string  (long time)