Defining rules for Jacobi functions?

I'd like to define some rules to convert some Jacobi elliptic
functions to other forms, and I don't know how. :-)

defrule(jsc, jacobi_sc(u,m), jacobi_sn(u,m)/jacobi_cn(u,m));

This works great:

apply1(sqrt(1+jacobi_sc(u,m)^2), jsc) ->

sqrt(1+jacobi_sn(u,m)^2/jacobi_cn(u,m)^2)

This doesn't work:

apply1(jacobi_sc(x/b, 1-b^2/a^2), jsc) ->

jacobi_sc(x/b,1-b^2/a^2)

How can I define such a rule?  Do I need to use
tellsimp/tellsimpafter?  I don't want it applied all the time; I want
it applied only when I explicitly say so.

Thanks,

Ray