Getting output of libsingular when it gives several objects

[mmarco]

In #19391 i am trying to move the method .invariant_generators() to libsingular. It works on the non modular case, but for the modular case, we need to call singular's invariant_ring function. This function returns three matrices, and we need the first two. But if i call it through libsingular, i only get the first one:

sage: from sage.libs.singular.function import singular_function
sage: import sage.libs.singular.function_factory
sage: sage.libs.singular.function_factory.lib('finvar.lib')
sage: inring = singular_function('invariant_ring')
sage: F=FiniteField(2)
sage: R.<x,y> = F[]
sage: m1 = matrix(R, 2, [0,1,1,0])
sage: inring(m1)
[x + y   x*y]

If you do the corresponding call within singular:

                     SINGULAR                                 /  Development

 A Computer Algebra System for Polynomial Computations       /   version 3-1-7

                                                           0<

 by: W. Decker, G.-M. Greuel, G. Pfister, H. Schoenemann     \   Aug 2013

FB Mathematik der Universitaet, D-67653 Kaiserslautern        \

> LIB "finvar.lib";

// ** loaded /home/mmarco/sage/local/share/singular/finvar.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/algebra.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/ring.lib (4.0.0.0,Jun_2013)

// ** loaded /home/mmarco/sage/local/share/singular/primdec.lib (4.0.1.1,Nov_2014)

// ** loaded /home/mmarco/sage/local/share/singular/absfact.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/triang.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/random.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/poly.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/inout.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/general.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/elim.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/matrix.lib (3-1-7-0,Sep_2013)

// ** loaded /home/mmarco/sage/local/share/singular/nctools.lib (3-1-7-0,Sep_2013)

> ring r=2,(x,y),dp;

> matrix A[2][2] = 0,1,1,0;

> matrix P,S,IS=invariant_ring(A);

> P;

P[1,1]=x+y

> S;

S[1,1]=xy

> IS;

IS[1,1]=1

How can i get these three matrices through libsingular?

[Kwankyu Lee]

I heard from the author of the libSingular interface that multiple return values were not considered in the initial design. So I guess you need to revise Sage's libSingular interface first.