We have DTEC and LDTC to detect the presence of an element, but no sensor to detect the absence of one.  Originally, I had the idea for a new sensor element that sparks adjacent conductors opposite to the side or corner that has no pariticles in line with it; if it had a ctype, only the element with that ctype would turn it off.  I was called the Hole detector, or HDTC, and it would function the exact opposite way as LDTC.  I scrapped the idea because it would be easier to give LDTC a new operator mode rather than add a whole new element:

LDTC tmp2 16; "Hole detection" mode.

In this operator mode, LDTC will spark a conductor only if there are no pariticles of any element in line the the side or corner opposite to the side or corner in contact with the conductor.  If it's ctype is set, it'll only turn off if the element matching it's ctype is in line with it.  If the tmp2 is set to 17 ("Invert" mode and "Hole detection" mode used in combination) then LDTC will turn off if any element other than the one that matches it's ctype is in line with it.

This feature would be useful in zamboni construction; a PSTN directed machine could push a LDTC-activated mechanism that, if it dectected the absence of an ICE particle, would trigger a CRAY or DRAY to place a particle of ICE onto the empty pixel.

I literally just mentioned LDTC's invert mode in the parentheses in the thread above.  Invert mode for LDTC doesn't detect the absense of an element, it detects the presence of any element except the one matching the LDTC's ctype.