The autobeam filter can be used to automatically add beams to notes according to the prevailing meter. Here is an example where the autobeam filter is being applied to the music, which contains one measure of 3/4 music and another in 6/8.
Try deleting the filter from the above Humdrum data and see what happens.
In the VHV editor, you can also type alt-c to “compile” the filter, which will result in the beam markers being added to the data in the editor. (The alt-c command will not work on this page, however).