integrating a stateful function with rk()

Ether, If what you want to do is have a parameterized vector field,
then Maxima lets you do this:

vf[a,b](x,y) := [a*x-b*y,b*xa*y];

Then

rk(vf[1,2](x,y), [x,y], .... )

will integrate the equations with a=1, b=2.

I am sure that you can do what you want in Maxima, but it may be that
your example is not clear as to what you want to do.

Leo