This paper presents a composition engine that handles User interface (UI) in the context of application composition. The aim is to detect and manage conflicts that may arise when composing UI driven by business needs. The originality of this composition engine is to reason at an Abstract level which simplifies the composition algorithm and makes it reusable and oblivious to technology. The composition engine is the core of the Alias framework that reduces the re-engineering efforts needed to obtain the UI of an application built by composition of smaller ones following the "programming in the large" paradigm.