The Layer class represent a list of packages.
This class represents an ordered set of layers, where each package on a given layer can only depend on packages defined in lower layers
this matcher checks that dependencies are satisfied for a given Layers definition
matcher for Layers declarations
a new Layer object. If a prefix is defined, it will apply to each given name
a new Layers object, composed of other layers. If a prefix is defined it will apply to each layer
this implicit definition allows to use a single string instead of a Layer object