need help with narray: how to return an array from a C-extension

Hi there,
I am writing my first C extension module to ruby, and I need
a little
help:
import of parameters works fine, but export of results not
yet:

static VALUE
myarr_faltung1(VALUE m, VALUE distribution, VALUE
lookuptable, VALUE
mu_shifted)
{
    struct NARRAY *a1, *a2;
    int mt, i;
    volatile VALUE v1, v2;

/* import from parameters seems to work */
    mt = NUM2INT(mu_shifted);
    v1 = na_cast_unless_narray(distribution,NA_DFLOAT);
    GetNArray(v1,a1);
    v2 = na_cast_unless_narray(lookuptable,NA_DFLOAT);
    GetNArray(v2,a2);

/* calculation seems to work */
    for (i=0; i<=1000; i++)
        *((double*)(a1->ptr+i)) +=
*((double*)(a2->ptr+mt-i));

/* PROBLEM: this does not work. QUESTION: how can I
overwrite the call
parameter "distribution"
    with a ruby array that contains the result a1->ptr of
the above
calculation? */
    distribution = na_to_array( a1->ref );

/* PROBLEM: the following does not work either. QUESTION:
how can I
return the result a1->ptr
    of the above calculation to the calling program? */
    return a1->ref ;
}

Thanks in advance, Joachim