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@@ -37,3 +37,32 @@ CALCULATIONS:
 For each new keyword falling in the same index entry, the subsequent page numbers must 
 be separated by some punctuation mark, ike the usual virgule.
 
+
+## Tags, labels and search engines
+
+The same way we do with latex to reference floating objects, we can create labels inside 
+files to highlight some specific information contained inside it. Inside files containing 
+code, for example, one could put labels inside commentaries. Let's take an example where a 
+FORTRAN .f90 file contains a FFT subroutine which we want to label for future use. We could
+then do:
+```fortran
+!   FAST FOURIER TRANSFORM :FFT
+```
+where we put the label :FFT. One can choose any label, they only rule is to follow 
+logic and be organized, do not just create random label forms each time labelling is needed.
+
+The above logic can also be applied to metadata (image files) by adding specific fields
+in the file header.
+
+Now, to search these labels inside files we need an appropriate engine. Inside linux, the easiest way
+is to `grep` or `pdfgrep` expression. During the course the following desktop applications were suggested: 
+- [DocFetcher](http://docfetcher.sourceforge.net/en/index.html)
+- [Recoll](https://www.lesbonscomptes.com/recoll/)
+- [Tracker](https://wiki.gnome.org/Projects/Tracker)
+
+for files containing text and metadata. For images, [Imagemagick](https://imagemagick.org/script/index.php) 
+and [Exiftoll](https://exiftool.org/) can also be employed.
+
+
+
+