Insert and extract generating new vector

Example:

// Create a new vector based on a and b, but avoid the final extract
rule(a,b,d) {
  t0 = undef();
  t0 = insert(t0,0,a);
  t0 = insert(t0,1,b);
  d = t0;
  return extract_low(t0);
} -> {
  t0 = undef();
  t0 = insert(t0,0,a);
  t0 = insert(t0,1,b);
  d = t0;
  return a;
}
 
rule(a,b,d) {
  t0 = undef();
  t0 = insert(t0,0,a);
  t0 = insert(t0,1,b);
  d = t0;
  return extract_high(t0);
} -> {
  t0 = undef();
  t0 = insert(t0,0,a);
  t0 = insert(t0,1,b);
  d = t0;
  return b;
}