  Lenses are a mathematical structure for maintaining consistency between a
pair of systems. In their ongoing research program, Johnson and Rosebrugh have
sought to unify the treatment of symmetric lenses with spans of asymmetric
lenses. This paper presents a diagrammatic approach to symmetric lenses between
categories, through representing the propagation operations with Mealy
morphisms. The central result of this paper is to demonstrate that the
bicategory of symmetric lenses is locally adjoint to the bicategory of spans of
asymmetric lenses, through constructing an explicit adjoint triple between the
hom-categories.
