Check diff(f, 2, x) behavior in SMT

[latot]

Hi, well actually this format don't is documented in SMT, so when we can check this we can remove or keep this feature from diff.

Related commit: #552

Exp of the problem: #523

Cya.

[cbm755]

Still supported in 2015b. I guess no hurry for us to to remove that feature. (anywhere there are notes next to the tests for it documenting it for the future.)