*2.2. Limitations*

We worked only with digital scientific articles, leaving out other important publications such as books and theses. Although unlikely, it is possible that we are leaving out some scientific articles published in paper format with no current online access (likely to be older than 1998). Another possibility is that some articles were not included in the results of our search engines; this happened with one of the author's research [12]. Another limitation can be that our proposed scales of time and space could be subject to debate.
