**labels**: --> Lisp Core - Taylor

t: taylor(sin(x+y),[x,y],0,5)

=> y+x-....

ratcoeff(t,x,1) => 0+...

but

ratcoeff(ratdisrep(t),x,1) => (y^4-12*y^2+24)/24

The ratdisrep case should be taken as the definition. If

we don't want to do that, it should at least give an

error, saying that it doesn't handle multivariate series.

It would be nicer, though, if ratcoeff took multiple

arguments, e.g.

ratcoeff(ratcoeff(t,x,1),y,0)

=> x+y

ratcoeff(t,[x,y],1) => x+y

ratcoeff(t,[x,y],2) => 0

ratcoeff(t,[x,y],3) =>

-(y^3+3*x*y^2+3*x^2*y+x^3)/6

but that's another matter....