the ladder is a lattice.
the simple alternating groups were read as a line. but the same word climbs PSL(2,7), PSL(2,11), PSL(2,13) — simple, non-solvable, none alternating. the gate is solvability, not simplicity.
two ladders crossing at A₅=PSL(2,5), A₆=PSL(2,9). the series is one strand.