Postgres: plpgsql combine ST_Overlaps and ST_Intersects

I have a question regarding creating a trigger function in postgres with 2 conditions:

when creating a linestring in QGIS their must either overlap nor intersect. So i wrote 2 function, each working with either ST_Overlaps or ST_Intersects.

CREATE OR REPLACE FUNCTION check_linesintersecting() RETURNS TRIGGER AS
$BODY$
DECLARE
BEGIN 
IF TG_OP = 'INSERT'
THEN
IF
(SELECT COUNT(*)
FROM
(SELECT gid
FROM line_layer AS t
WHERE st_intersects(NEW.geom, t.geom)) AS foo)
THEN
RAISE EXCEPTION 'Speicherung abgebrochen: Ueberschneidende Linien!';
RETURN NULL;
ELSE
RETURN NEW;
END IF;
ELSIF TG_OP = 'UPDATE'
THEN
IF
(SELECT COUNT(*)
FROM
(SELECT gid
FROM line_layer AS t
WHERE st_intersects(NEW.geom, t.geom)
AND (t.gid <> OLD.gid)) AS foo) > 0
THEN
RAISE EXCEPTION 'Speicherung abgebrochen: Ueberschneidende Linien!';
RETURN NULL;
ELSE RETURN NEW;
END IF;
END IF;

END;
$BODY$
LANGUAGE plpgsql;

All I had to to in oder to make it detect overlapping was to change the ST_Intersects into ST_Overlaps.

But how can I combinde ST_Overlaps and ST_Intersects? I've tried using the same loop right after each other but only the the first function then is working. Any clues? Thanks in advance!!