expr syntax question

From: John Wiersba (jrw32982_at_yahoo.com)
Date: 02/27/04 

Date: 27 Feb 2004 10:55:10 -0800

Never having been too comfortable with the requirements for getting
expr syntax correct, I came across the following anomaly:

% set tcl_patchLevel
8.0.4
% set y a
a
% set z b
b
% expr (0 == 0) ? a : b
syntax error in expression "(0 == 0) ? a : b"
% expr (0 == 0) ? $y : $z
a
...
% set tcl_patchLevel
8.3.3
% set y a
a
% set z b
b
% expr (0 == 0) ? a : b
syntax error in expression "(0 == 0) ? a : b"
% expr (0 == 0) ? $y : $z
syntax error in expression "(0 == 0) ? a : b"

Can someone please explain what's going on?

Thanks!
-- John Wiersba