\BV
A small, domain-specific esoteric programming language for transforming 64-bit integers.
A \BV program takes a 64-bit integer as its input and returns a 64-bit integer as the result. \BV has the usual bitwise operators and the fold function:
(fold e1 e2 f)
which applies function f to each of the eight bytes of e1 and the whole of e2.
\BV was defined in the task of the 2013 ICFP programming contest, in which the objective was to guess a secret \BV program by giving it inputs and receiving its outputs.
esoteric programming languages wiki entry.