derivate an equation decomposed in a tlist
[eqsimpl,reseq_ds]=deriv_eq(reseq,vari)
* reseq = an 'equation' tlist, the result of the breaking down of an equation into elementary bricks
* vari = a string, the name of a variable
* eqsimpl = the resulting symbolic derivative
* reseq_ds = the corresponding tlist collecting the structure of the derivative