[Help-smalltalk] Newbie Question: I got an error message

[Jumbler_Chi]

Dear All:
    I'm beginner.  I installed  gst 1.95.9 under Alpha server.
    I followed the chapter 5 of toturial from
http://www.gnu.org/software/smalltalk/gst-manual/gst_32.html#SEC54
to create 'Array' object , but it's invalid. Why ?

Have any preparation for starting smalltalk ?

PS. the  execution result is as below:
----------------------------------
$ gst -v
GNU Smalltalk version 1.95.9
written by Steve Byrne (address@hidden) and Paolo Bonzini (address@hidden)
Copyright 1988-92, 1994-95, 1999-2001 Free Software Foundation, Inc.

GNU Smalltalk comes with NO WARRANTY, to the extent permitted by law.
You may redistribute copies of GNU Smalltalk under the terms of the
GNU General Public License.  For more information, see the file named
COPYING.

Using default kernel path: /usr/users/jumbler/share/smalltalk/kernel
Using default image path: /usr/users/jumbler/share/smalltalk
$ gst
GNU Smalltalk Ready

st> x:= Array new: 20 !
stdin:1: assignment to undeclared variable x
st>
------------------------------------------------

Best Regards

Jumbler Chi