Regarding error : There is loop reference when define parameter in hspice

[srini.pes]

Hi to all

Am designing one spice compatible transistor, in that i need the error function erf.
i have mentioned it as a function
like

.PARAM t='1/(1+p*y)'

.PARAM erf= '(-((a1*t)+(a2*POW(t,2))+(a3*POW(t,3))+(a4*(POW(t,4)))+(a5*(POW(t,5))))*exp(-y*y))'

Now., it was showing this error., if i mentioned every parameter as a function like x instead of x.
Then there was no error.

Please suggest me some technique to avoid the use of everything as a function.