Great program! Like other users suggest, allow the user to combine search tags. For example, when a user clicks on a tag on the left, the searchlist is reduced to this selection, and the tag itself now appears in a different color, indicating it is selected. Now when the user clicks on another tag, the search list is further refined to those items that include both tags and the second tag appears also in the other color, so the user can clearly see which two tags he has combined. The ability to combine a search now seems only possible when the user types the names of the tags in manually, with a space and a + sign before it, which seems too limited.
Boostnote does this beautifully, take a look: https://camo.githubusercontent.com/4434304f608c83236c42bc5c3d93c93d97da1965/68747470733a2f2f692e696d6775722e636f6d2f646955555739622e676966