There is a download here: http://d20pro.com/downloads.php for Mac/Linux. This is just a folder that you save to your desktop. Instructions for use are in the readme file.

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Actually, d20pro doesn't stop you at all and doesn't enforce any rules for that matter. That's intentional, actually. d20pro is meant to be a tabletop aid - not a game in itself. There is a balance between adding automated processes and taking away player freedom. For example, what if your troll had a feat that allowed it to extend it's reach for Attacks of Opportunity? What if it was currently stunned? What if the DM ruled that it didn't notice the player? We wanted the system to allow anything because house rules, circumstances, and other factors like custom feats and magic could bend or break those rules any time. It is up to you and the DM to maintain rules integrity and adjudicate things like movement, reach, AoO, etc. The DM can simply move you back, type or say "STOP" and evoke the rule - just like at the table. Otherwise, we would be coding endless scenarios and never fully cover everything that can happen at the table. We hope that this free-form style of play is better for your game in the long run.

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