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.
Doc: $\var{bnf}$ being output by
 \kbd{bnfinit}, \var{sfu} by \kbd{bnfsunit}, gives the column vector of
 exponents of $x$ on the fundamental $S$-units and the roots of unity.
 If $x$ is not a unit, outputs an empty vector.