Thanks for pointing me to the link.
So, now one can create an asset, where the supply can only increase when somebody shorts the asset (going long BTS with leverage). And, once the contract is closed, the supply decreases. Am I correct?
correct
My other question is, is there a maintenance fee that asset creators need to keep paying the witnesses for posting feed price?
This of course is only valid for market pegged assets. User Issued
Assets do not have a price feed that needs to be maintained.
Further you can choose WHO exactly can produce a price feed and you also
get to pick the time a feed is valid (e.g. 24h, then you need one feed
every 24h).
The fee to produce a fees is extremely low, almost 0 simple because
the current witnesses need to already maintain a feed for a dozend of
smart coins (bitUSD, bitCNY, ....).
You can see all fees here:
http://cryptofresh.com/fees?asset=USDand fees for feeds are called "Asset publish feed".