coins[msg.sender] += 5;

func getInterest(){
    if( (coins[msg.sender].lastClainBlk + 100 ) < block.number ){
        coins[msg.sender].balance += coins[msg.sender].balance *RATE;
        coins[msg.sender].lastClainBlk = block.number
    }
}

assert(transactionsValid(transactions));
var new_head = hash(transactions, nonce, blockhead);
assert(new_head < threshold);
coins[msg.sender] += 5;
blockhead = new_head;