How to check if sequence already exists?

Question

I have the following sql code in a .sql file:

CREATE TABLE IF NOT EXISTS changes (
change_id integer NOT NULL,
change_date date NOT NULL default CURRENT_DATE,
change_file character varying(255),
description character varying(255)
);

CREATE SEQUENCE changes_change_id_seq
START WITH 1
INCREMENT BY 1
NO MINVALUE
NO MAXVALUE
CACHE 1
USING local;

 ALTER TABLE ONLY changes ALTER COLUMN change_id SET DEFAULT nextval('changes_change_id_seq'::regclass);

 INSERT INTO changes VALUES (DEFAULT,DEFAULT, 'temp.sql', 'test');

I need to make this script idempotent. I need to be able to run it multiple times without it failing.

Right now, it fails when I run it more than one time:

lab:/tmp# psql -U postgres -d widgets -f changes.sql 
psql:changes.sql:7: NOTICE:  relation "changes" already exists, skipping
CREATE TABLE
psql:chanegs.sql:15: ERROR:  relation "changes_change_id_seq" already exists
ALTER TABLE
INSERT 0 1

How can I modify the CREATE SEQUENCE and the ALTER TABLE statements to only run if the sequence doesn't already exist?

I've already tried the "IF NOT EXISTS" on the sequence but it bombs.