ADT

  element head(list* l) {  
  head  
    require( ! empty(l) );
    return l->e; 
 for both CONS and INTERVAL
    }
  
  list* tail(list* l) { 
  tail  
    require( ! empty(l) );
    switch( l->tag ) {
      case CONS: return l->next;
      case INTERVAL:
  	 return interval((l->e)+1,l->z);
  	}
    }