json ->> operator precedence

An interesting quirk:

# select CASE WHEN '{"a":null}'::jsonb->>'a' IS NULL THEN 'yes' ELSE 'no'
END;
case
------
yes

According to the precedence table
http://www.postgresql.org/docs/9.4/static/sql-syntax-lexical.html I would
expect ->> to come under "all other native and user-defined operators",
which would imply that this command should be testing whether 'a' IS NULL
and applying the result (false) to the json operator - at which point we
have

# SELECT CASE WHEN '{"a":null}'::jsonb->>false THEN 'yes' ELSE 'no' END;

and since

# SELECT '{"a":null}'::jsonb->>false;

returns NULL, the query is effectively:

# SELECT CASE WHEN NULL THEN 'yes' ELSE 'no' END;

which returns 'no'.

So the only way that we should get 'yes' is if the ->> has higher
precedence than 'IS NULL'.

OK, so be it; except if we assume that the reason is because the lex
analyzer sees '-' and assumes higher precedence than 'IS NULL' then you
would expect

SELECT '{"a":10}'::jsonb->>'a' - 5;

to return '5' - since left-to-right precedence would make ->> run before
the subtraction; however I get:

ERROR: invalid input syntax for integer: "a"
LINE 1: select '{"a":10}'::jsonb->>'a' - 5;

So what precedence level is ->> actually running at?

Or am I missing something?

Cheers

Geoff