Validate Binary Search Tree

Invariant: At node entry, (low,high) is the intersection of all ancestor constraints; a legal value lies inside it. In inorder form, every visited value is greater than the previous one.

candidate confidence: 0.733 · metadata_only

Invariant: dfs(u) returns a summary concerning only u's subtree and satisfying its parent contract; before return, every global candidate fully contained in that subtree has been considered.

candidate confidence: 0.603 · metadata_only