A reflective system needs metaknowledge to analyze and create knowledge easily. So, it is essential that we can express this metaknowledge in a convenient way, without bothering with the internal representation of knowledge. We define two functions, MATCH and CREATE, which receive the description of knowledge in a mathematical formalism. These functions are particularly useful when they are used together. They are compiled by expertises that use these functions themselves. They are used everywhere in the MACISTE system: to define declarative knowledge, to proceduralize it, to interpret it, to display it and so on.