diff --git a/src/collections.zig b/src/collections.zig
index 500d83f..1aa13e1 100644
--- a/src/collections.zig
+++ b/src/collections.zig
@@ -322,3 +322,155 @@ pub const Hash = struct {
         return true;
     }
 
+    pub fn len(db: *Db, allocator: Allocator, name: []const u8) !usize {
+        const p = try prefix(allocator, name);
+        defer allocator.free(p);
+        var it = try db.scanPrefix(allocator, p);
+        defer it.deinit();
+        var count: usize = 0;
+        while (try it.next()) |_| count += 1;
+        return count;
+    }
+
+    /// Returns every field/value pair currently stored under `name`. Caller
+    /// owns the returned slice and each entry's `field`/`value` buffers.
+    pub fn getAll(db: *Db, allocator: Allocator, name: []const u8) ![]HashField {
+        const p = try prefix(allocator, name);
+        defer allocator.free(p);
+
+        var results = std.ArrayList(HashField).init(allocator);
+        errdefer {
+            for (results.items) |f| f.free(allocator);
+            results.deinit();
+        }
+
+        var it = try db.scanPrefix(allocator, p);
+        defer it.deinit();
+        while (try it.next()) |entry| {
+            const field = try allocator.dupe(u8, entry.key[p.len..]);
+            errdefer allocator.free(field);
+            const value = try allocator.dupe(u8, entry.value);
+            try results.append(.{ .field = field, .value = value });
+        }
+        return results.toOwnedSlice();
+    }
+};
+
+// ---------------------------------------------------------------------
+// Set
+// ---------------------------------------------------------------------
+
+pub const Set = struct {
+    fn prefix(allocator: Allocator, name: []const u8) ![]u8 {
+        var out = std.ArrayList(u8).init(allocator);
+        errdefer out.deinit();
+        try out.append(nsTag);
+        try appendSegment(&out, "S");
+        try appendSegment(&out, name);
+        return out.toOwnedSlice();
+    }
+
+    fn memberKey(allocator: Allocator, name: []const u8, member: []const u8) ![]u8 {
+        var out = std.ArrayList(u8).init(allocator);
+        errdefer out.deinit();
+        try out.append(nsTag);
+        try appendSegment(&out, "S");
+        try appendSegment(&out, name);
+        try out.appendSlice(member);
+        return out.toOwnedSlice();
+    }
+
+    /// Returns true if the member was newly added (false if already present).
+    pub fn add(db: *Db, allocator: Allocator, name: []const u8, member: []const u8) !bool {
+        if (try isMember(db, allocator, name, member)) return false;
+        const k = try memberKey(allocator, name, member);
+        defer allocator.free(k);
+        try db.put(k, &.{});
+        return true;
+    }
+
+    /// Returns true if the member was present and has been removed.
+    pub fn remove(db: *Db, allocator: Allocator, name: []const u8, member: []const u8) !bool {
+        if (!try isMember(db, allocator, name, member)) return false;
+        const k = try memberKey(allocator, name, member);
+        defer allocator.free(k);
+        try db.delete(k);
+        return true;
+    }
+
+    pub fn isMember(db: *Db, allocator: Allocator, name: []const u8, member: []const u8) !bool {
+        const k = try memberKey(allocator, name, member);
+        defer allocator.free(k);
+        const raw = try db.get(allocator, k) orelse return false;
+        allocator.free(raw);
+        return true;
+    }
+
+    pub fn card(db: *Db, allocator: Allocator, name: []const u8) !usize {
+        const members_ = try members(db, allocator, name);
+        defer {
+            for (members_) |m| allocator.free(m);
+            allocator.free(members_);
+        }
+        return members_.len;
+    }
+
+    /// Returns every member currently in the set. Caller owns the slice and
+    /// each element.
+    pub fn members(db: *Db, allocator: Allocator, name: []const u8) ![][]u8 {
+        const p = try prefix(allocator, name);
+        defer allocator.free(p);
+
+        var results = std.ArrayList([]u8).init(allocator);
+        errdefer {
+            for (results.items) |m| allocator.free(m);
+            results.deinit();
+        }
+
+        var it = try db.scanPrefix(allocator, p);
+        defer it.deinit();
+        while (try it.next()) |entry| {
+            try results.append(try allocator.dupe(u8, entry.key[p.len..]));
+        }
+        return results.toOwnedSlice();
+    }
+
+    fn containsSorted(haystack: [][]const u8, needle: []const u8) bool {
+        for (haystack) |h| {
+            if (std.mem.eql(u8, h, needle)) return true;
+        }
+        return false;
+    }
+
+    /// Members present in every one of `names`. Caller owns the result.
+    pub fn intersect(db: *Db, allocator: Allocator, names: []const []const u8) ![][]u8 {
+        var results = std.ArrayList([]u8).init(allocator);
+        errdefer {
+            for (results.items) |m| allocator.free(m);
+            results.deinit();
+        }
+        if (names.len == 0) return results.toOwnedSlice();
+
+        const first = try members(db, allocator, names[0]);
+        defer {
+            for (first) |m| allocator.free(m);
+            allocator.free(first);
+        }
+
+        outer: for (first) |candidate| {
+            for (names[1..]) |other| {
+                if (!try isMember(db, allocator, other, candidate)) continue :outer;
+            }
+            try results.append(try allocator.dupe(u8, candidate));
+        }
+        return results.toOwnedSlice();
+    }
+
+    /// Members present in any of `names`, deduplicated. Caller owns the result.
+    pub fn unionOf(db: *Db, allocator: Allocator, names: []const []const u8) ![][]u8 {
+        var results = std.ArrayList([]u8).init(allocator);
+        errdefer {
+            for (results.items) |m| allocator.free(m);
+            results.deinit();
+        }
+