[newbie] how to expand function to original variables



I have the following code:
p(tau11,tau22):=tau11*tau22;
sigma(tau11,tau12,tau22):=tau11+tau12+tau22;
A:sqrt(p^2+2*p*sigma)-p;
hjmax:sqrt((1+A/p)/((1-A/sigma^2)^2+A/p));

I now would like to see hjmax written again as a function of tau11, tau12, 
tau22.
Can anyone here point me to which command to use for that purpose?

thanks in advance
hugo