  Uniform preorders are a class of combinatory representations of Set-indexed
preorders that generalize Pieter Hofstra's basic relational objects. An indexed
preorder is representable by a uniform preorder if and only if it has as
generic predicate. We study the $\exists$-completion of indexed preorders on
the level of uniform preorders, and identify a combinatory condition (called
'relational completeness') which characterizes those uniform preorders with
finite meets whose $\exists$-completions are triposes. The class of triposes
obtained this way contains relative realizability triposes, for which we derive
a characterization as a fibrational analogue of the characterization of
realizability toposes given in earlier work.
  Besides relative partial combinatory algebras, the class of relationally
complete uniform preorders contains filtered ordered partial combinatory
algebras, and it is unclear if there are any others.
