When(Where) does qual become a List ?

Hello all,

I see the following defintion of query_planner() in
optimizer/planner/planmain.c .

Plan * query_planner(Query *root,                         int command_type,                         List *tlist,
                List *qual)
 

Does this mean that qual is already a List when 
query_planner () is called ?

But I see the following code in query_planner()qual = cnfify((Expr *) qual, true);

Are Expr and List compatible ?
I could see such CAST in some places.

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp