accessing functions in the core

I would like to create functions in the core and then access it.
However, I do not know how to do it. Looks like all functions in the
core are not accessible from outside. For example ts_headline_opt which
is there in the core seems to be not present in pg_catalog.
This is the query I used to test:

testdb=# select * from pg_catalog.pg_proc where
proname='ts_headline_opt';

0 rows

Thanks,
-Sushant.