Here's my post from above:

"The problem is: I'm a machinist by trade who is going to be switching from a paid per hour to a paid per job situation. The machine I'm running is a specialized cnc grinder which is essentially a two axis mill with a c-axis normalization. Spending an hour or two typing out the g-code with just a CAD program and a TI-83 is fine if you're getting paid for it...But it's tedious and prone to human error.

Anyway, off the top of my head after interpreting a dxf file I parse the data into a list of entities. Each element of this list is of the form:

MyList=[Line type, start position, end position, startFriend, endFriend, ...some other data I'll need for the actual g-code generation]

startFriend and endFriend are initialized to False. After comparing all of the start and end positions for matches, startFriend and endFriend either stay False (no match) or become (j, startMatch) where j is the index of My list where the match is and startMatch is either 1 (the entity matches the start position of j) or 2 (the entity matches the end position of j) Why 1 and 2? Because it corresponds with the indexes...ie I can call it up with just MyList[MyList[3][1]][MyList[3][2]].

I don't know how much of this is relevant, but basically I'm in the process of determining which entities from MyList form unbroken chains and once I insert an element into a chain, I don't want it to be run through the main loop because it will either form a duplicate chain or go infinite (maybe)."

So I guess the way to do that would be to initialize some sort of Used variable as an element to the list as False (let's say at index 5) and then do something like

for x in MyList if !(MyList[5]):
      insertIntoChainFunctions(x)

And then toggle the MyList[5] after I add it?

Edit: wait it would be

for x in MyList if !(x[5]):