index a datatype

I have looked at the cube datataype in the contrib but I''m not sure
that I'm on the right way

I have found these functions:

-- support routines for indexing

CREATE FUNCTION cube_union(cube, cube) RETURNS cube
AS 'MODULE_PATHNAME' LANGUAGE 'c' with (isstrict);

CREATE FUNCTION cube_inter(cube, cube) RETURNS cube
AS 'MODULE_PATHNAME' LANGUAGE 'c' with (isstrict);

CREATE FUNCTION cube_size(cube) RETURNS float4
AS 'MODULE_PATHNAME' LANGUAGE 'c' with (isstrict);

and there are the same functions written in c in the file

/* cube_union */
NDBOX *
cube_union(NDBOX * box_a, NDBOX * box_b)
{
int i;
NDBOX *result;
NDBOX *a = swap_corners(box_a);
NDBOX *b = swap_corners(box_b);

if (a->dim >= b->dim)
{
result = palloc(a->size);
result->size = a->size;
result->dim = a->dim;
}
else
{
result = palloc(b->size);
result->size = b->size;
result->dim = b->dim;
}

/* swap the box pointers if needed */
if (a->dim < b->dim)
{
NDBOX *tmp = b;

b = a;
a = tmp;
}

/*
* use the potentially smaller of the two boxes (b) to fill in the
* result, padding absent dimensions with zeroes
*/
for (i = 0; i < b->dim; i++)
{
result->x[i] = b->x[i];
result->x[i + a->dim] = b->x[i + b->dim];
}
for (i = b->dim; i < a->dim; i++)
{
result->x[i] = 0;
result->x[i + a->dim] = 0;
}

/* compute the union */
for (i = 0; i < a->dim; i++)
result->x[i] = min(a->x[i], result->x[i]);
for (i = a->dim; i < a->dim * 2; i++)
result->x[i] = max(a->x[i], result->x[i]);

pfree(a);
pfree(b);

return (result);
}

Now my question is:

Is it easy to write an indexed datatype without touching the, let's say
internal code, of postgresql
Are there some problems when writing indexed datatypes?

Any information and suggestaions welcome

Ewald