Sort each statement about a binary search tree by whether it is an invariant of the structure.

An invariant: Every value in a node's left subtree is less than the node's value, Every node except the root has exactly one parent, An in-order traversal yields the values in sorted order

Not an invariant: The tree currently holds 12 values, The root currently holds the value 50

The ordering property, the single-parent rule and the sorted traversal survive every legal operation, so they are invariants. The current size and the current root value change with a single insertion, so they describe this moment rather than a rule, and a checker that asserted them would fire constantly on a perfectly healthy tree.