Functional index adding one

Hi all:

I'm trying to create a functional index into column position of token
table (see below). I want to make something like:

CREATE INDEX token_position_func
ON token (position+1);

but I get:

test=# CREATE INDEX token_position_func
test-# ON token (position+1);
ERROR: syntax error at or near "+"
LINE 2: ON token (position+1);

I read that I can do "ON function(column)" but, is there a built-in
function in PostgreSQL to do what I want (add one to the value) or have i
to build one to make this simple calculation?

CREATE TABLE doc(
id INT,
editorial VARCHAR,
CONSTRAINT doc_pk PRIMARY KEY (id)
);

CREATE TABLE token (
id INT,
id_doc INT,
token VARCHAR,
position INT,
CONSTRAINT foreign_doc FOREIGN KEY (id_do)
REFERENCES doc (identificador)
);

Regards,

Mario Barcala