'Mirror' Tool

Hello hello again [and congrats on the baby!] When making patterns that rely on mirror symmetry, it'd be a really neat to have a means to focus on 'half' of the design, so there's no worries concerning the other half of it. Miscounting beads on the screen without a ruler in place (which is another idea I had-- adding ruler/bead count to the pegboard space) is my paralysis demon sometimes.