How can I get a list/tuple of the variables in a symbolic function with the same ordering as when the function was defined? e.g. for the function below I would want (z,t) not the alphabetically ordered (t, z) I get with .variables() of .arguments(). The ordering has to be stored/used somewhere in sage because I can differentiate with respect to z and get D0(z,t) as an answer where the '0' corresponds to 'z'.
sage: var('z,t')
(z, t)
sage: f = function('u',z,t)
sage: print (f)
u(z, t)
sage: f.variables()
(t, z)
sage: f.arguments()
(t, z)
sage: f.diff(z)
D[0](u)(z, t)
sage: f.diff(t)
D[1](u)(z, t)
