Function: bnfissunit
Section: number_fields
C-Name: bnfissunit
Prototype: GGG
Help: bnfissunit(bnf,sfu,x): bnf being output by bnfinit (with flag<=2), sfu
 by bnfsunit, gives the column vector of exponents of x on the fundamental
 S-units and the roots of unity if x is a unit, the empty vector otherwise

