Pressing alt-m will replace Humdrum data in the text editor with its conversion to MEI data. The MEI data is usually generated behind the scenes in the process of converting Humdrum data into SVG images of music notation in VHV.
While MEI data is being displayed in the text editor, it can be edited and any changes will be reflected in the graphical notation window. In addition, clicking on objects in the notation will cause the text editor cursor to move to the parallel element in the MEI data; however, graphic editing commands will not work with the MEI data.
You can return the original Humdrum data to the text editor by pressing alt-h, but any changes made to the MEI data will be lost. Here is an example of typing Humdrum data into the text editor, and then typing alt-m to view the transcoded MEI data: