A declarative system for designing parts or assemblies of parts is based on a history-based design system (40), which has a storage format with primitives and operations; a modeled object is defined in the history based design system by an ordered sequence of operations and primitives. The declarative system has several functional features, which may be selected (48) and instantiated by a end user by providing a set of specifications (46) to a end-user interface (44). An instantiated functional feature provides contributions (50), each contribution being an ordered sequence of operations and primitives of the history-based design system. Contributions (50) are received by a seed structure (52). The seed structure is defined in a context-free grammar. The seed structure orders operations and primitives of received contributions provides an ordered sequence (54) of operations and primitives to the history based system. An editor (56) makes it possible to edit the seed structure in the grammar.