if I.leave() and I.find(buyer):
    if cofounder.wants(cofounder.shares > company.shares/2): 
      co-founder.buy(company.shares/2, buyer.shareprice)

As it was, our founding agreement stated that remaining founders could buy any number of shares at the same price a potential buyer paid. That allowed my co-founder to buy 1 share for $100 (or whatever) and gain majority control of the corporation, rather than forcing him to buy all my shares. This made it impossible (or at least improbable) to sell my shares to a 3rd party as it changed my 50% ownership to a minority position.