Function: bnrdisc
Section: number_fields
C-Name: bnrdisc0
Prototype: GDGDGD0,L,
Help: bnrdisc(a1,{a2},{a3},{flag=0}): absolute or relative [N,R1,discf] of
 the field defined by a1,a2,a3. [a1,{a2},{a3}] is of type [bnr],
 [bnr,subgroup], [bnf, module] or [bnf,module,subgroup], where bnf is as
 output by bnfclassunit (with flag<=2), bnr by bnrclass (with flag>0), and
 subgroup is the HNF matrix of a subgroup of the corresponding ray class
 group (if omitted, the trivial subgroup). flag is optional whose binary
 digits mean 1: give relative data; 2: return 0 if module is not the
 conductor

