  Bidirectional transformation, also called lens, has played important roles in
maintaining consistency in many fields of applications. A lens is specified by
a pair of forward and backward functions which relate to each other in a
consistent manner. The relation is formalized as a set of equations called lens
laws. This report investigates precise dependencies among lens laws: which law
implies another and which combination of laws implies another. The set of such
implications forms a complicated graph structure. It would be helpful to check
a well-definedness of bidirectional transformation in a lightweight way.
