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