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diff --git a/tvix/castore/src/nodes/mod.rs b/tvix/castore/src/nodes/mod.rs
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+++ b/tvix/castore/src/nodes/mod.rs
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+//! This holds types describing nodes in the tvix-castore model.
+mod directory;
+mod symlink_target;
+
+use crate::B3Digest;
+pub use directory::Directory;
+pub use symlink_target::{SymlinkTarget, SymlinkTargetError};
+
+/// A Node is either a [DirectoryNode], [FileNode] or [SymlinkNode].
+/// Nodes themselves don't have names, what gives them names is either them
+/// being inside a [Directory], or a root node with its own name attached to it.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Node {
+    /// A DirectoryNode is a pointer to a [Directory], by its [Directory::digest].
+    /// It also records a`size`.
+    /// Such a node is either an element in the [Directory] it itself is contained in,
+    /// or a standalone root node.
+    Directory {
+        /// The blake3 hash of a Directory message, serialized in protobuf canonical form.
+        digest: B3Digest,
+        /// Number of child elements in the Directory referred to by `digest`.
+        /// Calculated by summing up the numbers of nodes, and for each directory,
+        /// its size field. Can be used for inode allocation.
+        /// This field is precisely as verifiable as any other Merkle tree edge.
+        /// Resolve `digest`, and you can compute it incrementally. Resolve the entire
+        /// tree, and you can fully compute it from scratch.
+        /// A credulous implementation won't reject an excessive size, but this is
+        /// harmless: you'll have some ordinals without nodes. Undersizing is obvious
+        /// and easy to reject: you won't have an ordinal for some nodes.
+        size: u64,
+    },
+    /// A FileNode represents a regular or executable file in a Directory or at the root.
+    File {
+        /// The blake3 digest of the file contents
+        digest: B3Digest,
+
+        /// The file content size
+        size: u64,
+
+        /// Whether the file is executable
+        executable: bool,
+    },
+    /// A SymlinkNode represents a symbolic link in a Directory or at the root.
+    Symlink {
+        /// The target of the symlink.
+        target: SymlinkTarget,
+    },
+}