Error when following a documentation

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/10/static/plpgsql-cursors.html
Description:

As specified On page
https://www.postgresql.org/docs/10/static/plpgsql-cursors.html#PLPGSQL-CURSOR-OPENING
i am trying to run below commands
-------------------------------------------------
DECLARE
key integer;
curs4 CURSOR FOR SELECT * FROM tenk1 WHERE unique1 = key;
BEGIN
key := 42;
OPEN curs4;
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but i am getting below errors, please help
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postgres=#
postgres=# SELECT VERSION();
version

-----------------------------------------------------------------------------------------
----------------
PostgreSQL 10.4 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.4.7
20120313 (Red Hat 4.
4.7-18), 64-bit
(1 row)

postgres=# DECLARE
postgres-# key integer;
ERROR: syntax error at or near "integer"
LINE 2: key integer;
^
postgres=# curs4 CURSOR FOR SELECT * FROM tenk1 WHERE unique1 = key;
ERROR: syntax error at or near "curs4"
LINE 1: curs4 CURSOR FOR SELECT * FROM tenk1 WHERE unique1 = key;
^
postgres=# BEGIN
postgres-# key := 42;
ERROR: syntax error at or near "key"
LINE 2: key := 42;
^
postgres=# OPEN curs4;
ERROR: syntax error at or near "OPEN"
LINE 1: OPEN curs4;
^
postgres=#

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