Solidity PLASMA exit queue

[snjax]

Plasma exit queue is the queue of tuples (uint hash, uint SFT, address next) ordered by SFT.

We need to construct the queue with last element border (tail element's SFT is infinity) and first element border (first element's SFT is zero) and the following methods:

function _insert(address posBefore, uint hash, uint SFT) private returns (bool);
function _remove(address posBefore, address posCurrent) private returns (bool);

The contract must check that the queue is still sorted after element insertion complete.