This is a minimalistic definition of a contravariant functor called Tracer
intended to express the pattern common to logging, tracing, monitoring, etc.
in which domain-specific values are provided to domain-agnostic processors.
The Contravariant instance on Tracer m is the mechanism by which these
general-purpose tracers are adapted to stand in where application-specific
tracers are required.
Find documentation and examples in the source file.