# This function returns the union of two vectors, except that it is
# not sorted but instead returned in the original order.

combine = function( a; b )
{
    local( ab; ab_eid; u; v; i; r );

    # Append the vectors, and save a copy of their labels.

    ab = vector( a ), vector( b );
    if ( !ab.ne ) { return ab; }
    ab_eid = ab.eid;
    ab.eid = NULL;

    # Get rid of duplicates.

    u = set( ab );
    v = 1:u.ne;
    for ( i in 1:u.ne ) { v[i] = find( u[i]; ab )[1]; }
    v = sort(v);
    r = ab[v];
    if ( ab_eid != NULL ) { r.eid = ab_eid[v]; }

    return r;
};