  Applicative bisimilarity is a coinductive characterisation of observational
equivalence in call-by-name lambda-calculus, introduced by Abramsky in 1990.
Howe (1989) gave a direct proof that it is a congruence. We propose a
categorical framework for specifying operational semantics, in which we prove
that (an abstract analogue of) applicative bisimilarity is automatically a
congruence. Example instances include standard applicative bisimilarity in
call-by-name and call-by-value $\lambda$-calculus, as well as in a simple
non-deterministic variant.
