## 3.6. Meta-keywords attached to the keywords

This section explains the meaning of the meta -keywords attached to the keywords of the catalogue. Keywords are usually associated with one single meta -keyword, except some that are linked to the meta -keyword modelling exercises and to one other meta -keyword like modelling or puzzles (e.g., see for instance the keywords magic series or degree of diversity of a set of solutions). For each meta -keyword it first gives the list of keywords using the corresponding meta -keyword and then defines the meta -keyword. At present the following meta -keywords are in use.