Creating a function if it's not there in Postgres

I use a set of DDL scripts to automatically create the database for an app.

I need to create certain functions (from contrib), if they're not there
already. If they are there, I don't want to DROP or REPLACE them, since
they may be used in certain indexes and triggers.

Is there a way to only CREATE a function if it is not defined already?
Is there a simple way to see if the function is defined?

(I don't want to just try creating it and letting it fail, since the DDL
runs in a transaction).

Thank you.