Function: rnfhnfbasis
Section: number_fields
C-Name: rnfhnfbasis
Prototype: GG
Help: rnfhnfbasis(bnf,x): given an order x as output by rnfpseudobasis,
 gives either a true HNF basis of the order if it exists, zero otherwise.
Doc: given $\var{bnf}$ as output by
 \kbd{bnfinit}, and either a polynomial $x$ with coefficients in $\var{bnf}$
 defining a relative extension $L$ of $\var{bnf}$, or a pseudo-basis $x$ of
 such an extension, gives either a true $\var{bnf}$-basis of $L$ in upper
 triangular Hermite normal form, if it exists, and returns $0$ otherwise.
