So now it just tries to keep track of who's where and when. If someone leaves by the school-gates during classes, their gait is checked to see if it kinda-sorta matches any student gait and if it does, whoop-whoop-whoop, ring the alarm!

Chavez High is ringed with gravel walkways. I like to keep a couple handsful of rocks in my shoulder-bag, just in case. I silently passed Darryl ten or fifteen pointy little bastards and we both loaded our shoes.

    - Cory Doctorrow, Little Brother: https://craphound.com/littlebrother/Cory_Doctorow_-_Little_Brother.pdf