FUNCTION :  Bord - Calculates a order of a theta-function at a cusp

CALLING SEQUENCE :  Bord(n,r,a,c)        

PARAMETERS :      n,r,a,c - positive integers such that (a,c)=1,
                            and n does not divide r.

SYNOPSIS :   Calculates ord(f[n,r],a/c) where 
             f[n,r] = q^( (n-2r)^2/(8n) )*(q^r,q^(n-r),q^n;q^n)oo
             as defined on p.277 of Biagioli. See Lemma 3.2 of Biagioli
             Assumptions: (a,c)=1 and n does not divide r.
             This function is used by the proc getacuspord for
             calculating the order at cusps of a generalized eta-quotient.
EXAMPLES :   

>  Bord(3,1,1,0);
                                1/24

> Bord(25,5,1,5);
                                 1/8

> Bord(5,1,1,5); 
                                9/40

DISCUSSION : Bord(3,1,1,0) = ord(eta(tau),oo)

SEE ALSO :  getacuspord