I am reading up on Sonic: Zero Knowledge SNARKS from Linear - Sized Universal And Updateable Structured Reference Strings.

An excerpt:

> 
> For soundness, we do not use the standard definition of special soundness because our verifier provides two challenges, but rather the generalized not of witness-extended emulation. 
> 
> 
> 

Can anyone provide both a simple and terse explanation of the term
 ***"witness-extended emulation"***