  We construct a category of fibrant objects $\mathbb{C}\langle P\rangle$ in
the sense of K. Brown from any indexed frame (a kind of indexed poset
generalizing triposes) $P$, and show that its homotopy category is the
Barr-exact category $\mathbb{C}[P]$ of partial equivalence relations and
compatible functional relations. In particular this gives a presentation of
realizability toposes as homotopy categories.
  We give criteria for the existence of left and right derived functors to
functors $\mathbb{C}\langle\Phi\rangle : \mathbb{C}\langle P\rangle\to
\mathbb{C}\langle Q\rangle$ induced by finite-meet-preserving transformations
${\Phi} : P \to Q$ between indexed frames.
