how to dump tree in pt.c:tsubst?

[Larry Evans]

How does one dump the trees in pt.c:tsubst in some hunan readable 
format.  I've tried:

   struct dump_info di;
   di.stream=stderr;
   cp_dump_tree(&di, args);
   cp_dump_tree(&di, arg);
   cp_dump_tree(&di, t);

however, no ouput is produced because the TREE_CODE for each of
the 2nd args doesn't match anything in the switch statements
inside cp_dump_tree (defined in cp/dump.c).

TIA.