FUNCTION :  eta2jac - convert from ETA to JAC notation 

CALLING SEQUENCE :  eta2jac()
                    eta2jac(etaexp)
                    
PARAMETERS :      etaexp - expression involving ETAnm and ETAn

SYNOPSIS :   
   This function converts etaexp into JAC-prod form;ie.
   in terms of JAC(a,b,infinity).

EXAMPLES :   
> with(qseries):
> with(thetaids):

> eta2jac();
-------------------------------------------------------------
eta2jac(getaexp)                                            
   getaexp is expression involving ETAnm(d,g) and ETAn(b).
   ETAnm(d.g)=eta[d,g](tau) amd ETAn(b)=eta(b*tau). 
   This function converts getaexp into JAC-prod form;ie.  
   in terms of JAC(a,b,infinity).                         
-------------------------------------------------------------

> EP:=ETAnm(5,1)/ETAnm(5,2):
> EP1:=ETAn(1):
> eta2jac(EP);
                                                          1/5
                                     JAC(1, 5, infinity) q
                                     ------------------------
                                       JAC(2, 5, infinity)
 
> eta2jac(EP1);
                                    (1/24)
                                   q       JAC(0, 1, infinity)
 
> eta2geta(EP);
                                          eta[5, 1](tau)
                                          --------------
                                          eta[5, 2](tau)
 
> eta2geta(EP1);
                                             eta(tau)
 
>  EP:=ETAnm(5,1);
                               EP := ETAnm(5, 1)

>  eta2geta(EP);
                                eta[5, 1](tau)

>  JP:=eta2jac(EP);
                                                   1/60
                              JAC(1, 5, infinity) q
                        JP := -------------------------
                                 JAC(0, 5, infinity)

>  jac2eprod(JP);
                                  GETA(5, 1)


SEE ALSO :  eta2geta, jac2eprod