  We present a higher well-ordering principle which is equivalent (over
Simpson's set theoretic version of $\text{ATR}_0$) to the existence of
transitive models of Kripke-Platek set theory, and thus to
$\Pi^1_1$-comprehension. This is a partial solution to a conjecture of
Montalb\'an and Rathjen: partial in the sense that our well-ordering principle
is less constructive than demanded in the conjecture.
