use the bit varying(n)

Hello,
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when I learn how to use the varbit(n) datatype, I got the error:
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test=3D# create table testdt ( b varbit(2) );
ERROR:  parser: parse error at or near "("

but can use as below :
test=3D# create table testdt ( b bit varying(2) );
CREATE

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The version is:
test=3D# select version();
                           version=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20
-------------------------------------------------------------
 PostgreSQL 7.2.3 on i686-pc-linux-gnu, compiled by GCC 2.96

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