Rewrote the union function to signal a specific condition when one of the lists is not a proper list.
Rewrote the union function to signal a specific condition
when one of the lists is not a proper list.

vtomole/SICL,clasp-developers/SICL,vtomole/SICL,vtomole/SICL,clasp-developers/SICL,clasp-developers/SICL,vtomole/SICL,clasp-developers/SICL