mov'ing an empty register to a memory array cell is ignored, either with direct or indirect indexing. see test program. also clr on a single memory array cell is not allowed. neither is mov'ing an empty array cell into that cell.
with none of those ways working, there's no way to clear one signal cell in a memory array. if there is a way at all, it should be documented. if not, then that should be documented.
test program:
clr
mov mem1[1] 1[virtual-signal=signal-R]
mov mem1[2] 1[item=iron-plate]
mov r1 1[item=iron-ore]
clr r2
mov mem1@1 r2 # should have cleared mem1[1], didn't.
nop
mov mem1[1] r2 # should have cleared mem1[1], didn't.
nop
clr mem1[1] # error, not allowed
