"If you have a Villain fielded, pay [1 Shield] to give a target character -1A and -1D (knock out those with zero defense)."
Most trigger abilities seem to have discrete triggers that can occur multiple times in a turn.
Doctor Doom reads like a trigger ability (If/when X, pay Y to get effect Z), but the 'trigger' (if you have a Villain fielded) cannot trigger multiple times in a turn.
So is the if statement here describing a trigger that allows for 0 or 1 uses on a turn or simply there as a check of the game state, similar to "if active" abilities?