It is common sense to hold a position of 1-1 in the futures market:
A opens a long position of 5 BTC, closes 5 long positions of BTC, and the position remains unchanged
A opens a long position of 5 BTC, opens 5 short positions of BTC, and the position is +5
A opens a long position of 5 BTC, opens 2.5 short positions of BTC, and C closes a long position of 2.5, and the position is +2.5