  Feature modeling has been a very popular approach for variability management
in software product lines. Building a feature model requires substantial domain
expertise, however, even experts cannot foresee all future possibilities.
Changing requirements can force a feature model to evolve in order to adapt to
the new conditions. Feather is a language to describe model transformations
that will evolve a feature model. This article presents the structure and
foundations of Feather. First, the language elements, which consist of
declarations to characterize the model to evolve and commands to manipulate its
structure, are introduced. Then, semantics grounding in feature model
properties are given for the commands in order to provide precise command
definitions. Next, an interpreter that can realize the transformations
described by the commands in a Feather script is presented. Finally,
effectiveness of the language is discussed using two realistic examples, where
one of the examples includes a system from a dynamic environment and the other
employs a system that has a large feature model containing 1,227 features.
