The Semantic Web is founded on a number of Formal Languages (FL) whose benefits are precision, lack of ambiguity, and ability to automate reasoning tasks such as inference or query answering. This however poses the challenge of mediation between machines and users because the latter generally prefer Natural Languages (NL) for accessing and authoring knowledge. In this paper, we introduce the N<A>F design pattern based on Abstract Syntax Trees (AST), Huet’s zippers and Montague grammars to zip together a natural language and a formal language. Unlike question answering, translation does not go from NL to FL, but as symbol N<A>F suggests, from ASTs (A) of an intermediate language to both NL (N<A) and FL (A>F). ASTs are built interactively and incrementally through a user-machine dialog where the user only sees NL, and the machine only sees FL.