Store the sign, not the number

Say two players take turns, A and B, and you need to track whose turn it is.

Most people number them 1 and 2. Swapping is 3 minus the turn, which is fine, but finding out whose turn it is means subtracting 2 before you can TAC.

cell  contents  notes
 10      030    INP 30  cells 10-13: whose turn, 1 for A and 2 for B; 3 - turn swaps them
 11      132    CLA 32
 12      730    SUB 30
 13      630    STO 30
 14      731    SUB 31  cells 14-19: which is it? a SUB first, then the TAC
 15      318    TAC 18
 16      531    OUT 31  B's turn: print 2
 17      900    HRS 00
 18      500    OUT 00  A's turn: print 1
 19      900    HRS 00
 30      000
 31      002
 32      003

Step through the natural way in Cardigan

Number them 0 and -1 instead. Swapping becomes -1 minus the turn, and finding out whose turn it is is the TAC on its own. Any time you're choosing between two things, you can store the choice as a sign.

cell  contents  notes
 10      030    INP 30  cells 10-18: whose turn, 0 for A and -1 for B; -1 - turn swaps them
 11      131    CLA 31
 12      730    SUB 30
 13      630    STO 30
 14      317    TAC 17  one TAC reads it
 15      500    OUT 00  A's turn: print 1
 16      900    HRS 00
 17      532    OUT 32  B's turn: print 2
 18      900    HRS 00
 30      000
 31     -001
 32      002

Step through the CARDIAC way in Cardigan

Both swap A's turn to B's and print 2. Numbering 1 and 2 takes 13 cells and runs 8 instructions; 0 and -1 takes 12 cells and runs 7.

From the tips in cardiac-code on GitHub. All the tips.