I'm reading [this paper](http://www.cpmt.org/scv/meetings/chua.pdf) by Chua on the memristor. He claims that

> 
> Using a mutator, a memristor with any prescribed `phi`-`q` curve can be realized
> 
> 
> 

(Here `phi` is the flux-linkage and `q` the charge.) The obvious [search for mutator](http://lmgtfy.com/?q=mutator%20electronics) is less than helpful.

What is a mutator in this context?