[Cryptography] cheap sources of entropy

[dan at geer.org]

It was said that:

> One well-calibrated well-defended well-monitored entropy source
> makes incomparably more sense than an arbitrarily complicated
> conglomeration of sucky sources.

Recalibrating first principles for a moment, please.  My understanding
is that a mix of N bit streams will be truly unpredictable if any 1 of
the N bit streams is truly unpredictable.

If that is incorrect, what am I missing?  (RTFM is entirely acceptable
and even gracious if accompanied by a pointer to TFM to R.)

--dan