concepts
Everything is a set
a set is a list of elements, and the list means intersection: wine together with red is the set of things that are both
an element can be a set in its own right, so sets nest without limit
there is no second kind of thing here — no folder, no tag, no document
Nesting is intersecting
a line indented under another inherits every element of the line above
so the child is a subset of its parent, and that subset relation is the whole of the structure
Tab and Shift+Tab are therefore not formatting; they add and remove elements
Expansion
expanding a set shows every set that contains it
each row prints only what it adds to its parent, so you read the differences rather than the repetitions
the same set turns up under every element it holds — nothing has one correct home
Names find each other
a name is a single element across the whole space
write the same word in two places and the two places are already connected: there is no link to make and none to maintain
Order is only display
sets are unordered
dragging a row changes what you see and never what is true, which is why the same material can sit in two arrangements at once