From Zappa Wiki Jawaka
Iannis Xenakis, (May 29, 1922 – February 4, 2001) was a Greek composer and architect and major contributor to musical modernism.
Den Simms: "Here's another one. In Bremen, the secret word was "Xenakis mbl, mbl, mbl, mbl".  I know Xenakis as being the composer, I believe he's Greek, right?"
Frank Zappa: Well, he's a Greek composer who lives in France, and he is an architect, by the way. That's really how he earns his living.
Den Simms: "What was the reference of his name being the secret word for that night?"
Frank Zappa: "OK. Remember I told ya I was doing a documentary for German television? It's all about serious composition, and the guy who did it is a guy named Henning Lohner, and Henning had been trying to reach me for months about doing this documentary. And he came to Bremen before the show to meet with me, and he told me about the other things that he'd done. He'd done a documentary on Karlheinz Stockhausen, John Cage, and Xenakis, and on two people in contemporary theater, whose names escape me right now. We were talking backstage about Xenakis, prior to the show. That's how ..."
Den Simms: "It just was ..."
Frank Zappa: "Yeah. But see the mbl, mbl, mbl, all that stuff, the early Xenakis pieces make use of repeated notes like that, as part of the texture. Different instruments repeating notes at different rates against each other. It's called "stochastic" composition."
Den Simms: "That was another question going along with that, 'cause I can remember that word being used, and I wondered what "stochastic" meant."
Frank Zappa: "Xenakis is the stochastic school of composition. And it's spelled S-T-O-C-H-A-S-T-I-C."