dual lambda sensors

[Mattias Nyberg]

I've noticed that several cars have two lambda sensors, one
before the catalyst and the other after. I've heard that one
reason for this is to improve the closed loop fuel control.
How does it work? What is gained by using the second lambda
sensor? Isn't the oxygen that the first lambda sensor measures
used up in the catalyst so there is no oxygen left for the
second lambda sensor to measure?

Mattias Nyberg