Leave a fee
A trade of $STIG against IMD leaves a fee, and that fee is the trace. At the start it is 10%, paid in IMD. A sell adds 20% that fades to zero over the first 30–60 minutes and joins the seat fund. After it fades, a sell pays the same rate as a buy.
Each ant that joins lowers the fee by 0.07%. It falls in a straight line from 10% with none to 3% at 100 ants, and it stays there. It does not rise if an ant later stops. The 1% burn stays the whole way.
