Work in progress
https://github.com/lukso-io/rico/blob/master/refundable.js
In the specification, my reading says that at any time, the amount available for withdrawal is:
this.balance * (timeSinceLastWithdrawal / totalDispersalTime)
Since users can impact the value of this.balance by refunding or buying tokens, this means that the wallet owner is incentivised to order their transactions relative to token holders (e.g. it is better to mine your transaction just after a purchase (buy) of tokens transaction has gone through, and conversely just before a refund of tokens goes through).
In this implementation I made the withdrawal rate constant and initialised it to the initial (and only) wallet deposit divided by the specified dispersalLength (number of blocks over which the deposit can be claimed).
This means it is possible to fully drain the contract before the dispersalLength has passed (if token holders choose to refund some of their tokens) but means there is no advantage to the wallet owner of ordering their transactions relative to others.