In order to unify our internal exchange and communication about transformations, we propose TrML (Transformation Modeling Language), a unified UML notation to design model transformations. This proposal aims to reify the synthesis of existing notations dedicated to transformation modeling. TrML is independent from implementation details and could be adapted to several transformation engines. To let TrML run on top of existing engine, we transform TrML model to a model accepted by the engine. But, which language should we use for the first transformation? TrML, the targeted engine or another one? In this article we will describe how we bootstrap our new language on top of existing transformation engines.