Spriteflags

Inside actor, state, and event code,
spriteflags <value>
sets the htflags value for the current sprite.

Outside of actors, states, and events,
spriteflags <picnum> <value>
sets the same flags per-tile. The per-tile value is XORed with the per-sprite value, i.e. the setting of one to "true" reverses the other.