Index: api/const.go
==================================================================
--- api/const.go
+++ api/const.go
@@ -117,27 +117,31 @@
 	CommandRefresh       = Command("refresh")
 )
 
 // Supported search operator representations.
 const (
-	BackwardDirective = "BACKWARD" // Backward-only context
+	BackwardDirective = "BACKWARD" // Backward-only context / thread
 	ContextDirective  = "CONTEXT"  // Context directive
 	CostDirective     = "COST"     // Maximum cost of a context operation
-	ForwardDirective  = "FORWARD"  // Forward-only context
+	DirectedDirective = "DIRECTED" // Context/thread collection can have general directions
+	FolgeDirective    = "FOLGE"    // Folge thread
+	ForwardDirective  = "FORWARD"  // Forward-only context / thread
 	FullDirective     = "FULL"     // Include tags in context
 	IdentDirective    = "IDENT"    // Use only specified zettel
 	ItemsDirective    = "ITEMS"    // Select list elements in a zettel
-	MaxDirective      = "MAX"      // Maximum number of context results
+	MaxDirective      = "MAX"      // Maximum number of context / thread results
 	MinDirective      = "MIN"      // Minimum number of context results
 	LimitDirective    = "LIMIT"    // Maximum number of zettel
 	OffsetDirective   = "OFFSET"   // Offset to start returned zettel list
 	OrDirective       = "OR"       // Combine several search expression with an "or"
 	OrderDirective    = "ORDER"    // Specify metadata keys for the order of returned list
 	PhraseDirective   = "PHRASE"   // Only unlinked zettel with given phrase
 	PickDirective     = "PICK"     // Pick some random zettel
 	RandomDirective   = "RANDOM"   // Order zettel list randomly
 	ReverseDirective  = "REVERSE"  // Reverse the order of a zettel list
+	SequelDirective   = "SEQUEL"   // Sequel / branching thread
+	ThreadDirective   = "THREAD"   // Both folge and Sequel thread
 	UnlinkedDirective = "UNLINKED" // Search for zettel that contain a phase(s) but do not link
 
 	ActionSeparator = "|" // Separates action list of previous elements of query expression
 
 	KeysAction     = "KEYS"     // Provide metadata key used

Index: client/client_test.go
==================================================================
--- client/client_test.go
+++ client/client_test.go
@@ -25,27 +25,21 @@
 	"t73f.de/r/zsc/domain/id"
 )
 
 func TestZettelList(t *testing.T) {
 	c := getClient()
-	_, err := c.QueryZettel(context.Background(), "")
-	if err != nil {
+	if _, err := c.QueryZettel(context.Background(), ""); err != nil {
 		t.Error(err)
-		return
 	}
 }
 
 func TestGetProtectedZettel(t *testing.T) {
 	c := getClient()
-	_, err := c.GetZettel(context.Background(), id.ZidStartupConfiguration, api.PartZettel)
-	if err != nil {
-		if cErr, ok := err.(*client.Error); ok && cErr.StatusCode == http.StatusForbidden {
-			return
-		} else {
+	if _, err := c.GetZettel(context.Background(), id.ZidStartupConfiguration, api.PartZettel); err != nil {
+		if cErr, ok := err.(*client.Error); !ok || cErr.StatusCode != http.StatusForbidden {
 			t.Error(err)
 		}
-		return
 	}
 }
 
 func TestGetSzZettel(t *testing.T) {
 	c := getClient()

Index: domain/id/id.go
==================================================================
--- domain/id/id.go
+++ domain/id/id.go
@@ -87,11 +87,10 @@
 
 	// WebUI image zettel are in the range 40000..49999
 	ZidEmoji = Zid(40001)
 
 	// Other sxn code zettel are in the range 50000..59999
-	ZidSxnPrelude = Zid(59900)
 
 	// Predefined Zettelmarkup zettel are in the range 60000..69999
 	ZidRoleZettelZettel        = Zid(60010)
 	ZidRoleConfigurationZettel = Zid(60020)
 	ZidRoleRoleZettel          = Zid(60030)

Index: domain/id/idset/idset.go
==================================================================
--- domain/id/idset/idset.go
+++ domain/id/idset/idset.go
@@ -131,12 +131,16 @@
 // Both sets can be modified by this method. One of them is the set returned.
 // It contains the intersection of both, if s is not nil.
 //
 // If s == nil, then the other set is always returned.
 func (s *Set) IntersectOrSet(other *Set) *Set {
-	if s == nil || other == nil {
-		return other.Clone()
+	if s == nil {
+		return other // no other.Clone(), since other != nil, i.e. "not found"
+	}
+	if other == nil {
+		s.seq = s.seq[:0]
+		return s
 	}
 	topos, spos, opos := 0, 0, 0
 	for spos < len(s.seq) && opos < len(other.seq) {
 		sz, oz := s.seq[spos], other.seq[opos]
 		if sz < oz {

Index: domain/meta/meta.go
==================================================================
--- domain/meta/meta.go
+++ domain/meta/meta.go
@@ -144,18 +144,16 @@
 	KeyForward      = "forward"
 	KeyLang         = "lang"
 	KeyLicense      = "license"
 	KeyModified     = "modified"
 	KeyPrecursor    = "precursor"
-	KeyPredecessor  = "predecessor"
 	KeyPrequel      = "prequel"
 	KeyPublished    = "published"
 	KeyQuery        = "query"
 	KeyReadOnly     = "read-only"
 	KeySequel       = "sequel"
 	KeySubordinates = "subordinates"
-	KeySuccessors   = "successors"
 	KeySummary      = "summary"
 	KeySuperior     = "superior"
 	KeyURL          = "url"
 	KeyUselessFiles = "useless-files"
 	KeyUserID       = "user-id"
@@ -172,11 +170,10 @@
 	registerKey(KeySyntax, TypeWord, usageUser, "")
 
 	// Properties that are inverse keys
 	registerKey(KeyFolge, TypeIDSet, usageProperty, "")
 	registerKey(KeySequel, TypeIDSet, usageProperty, "")
-	registerKey(KeySuccessors, TypeIDSet, usageProperty, "")
 	registerKey(KeySubordinates, TypeIDSet, usageProperty, "")
 
 	// Non-inverse keys
 	registerKey(KeyAuthor, TypeString, usageUser, "")
 	registerKey(KeyBack, TypeIDSet, usageProperty, "")
@@ -191,11 +188,10 @@
 	registerKey(KeyForward, TypeIDSet, usageProperty, "")
 	registerKey(KeyLang, TypeWord, usageUser, "")
 	registerKey(KeyLicense, TypeEmpty, usageUser, "")
 	registerKey(KeyModified, TypeTimestamp, usageComputed, "")
 	registerKey(KeyPrecursor, TypeIDSet, usageUser, KeyFolge)
-	registerKey(KeyPredecessor, TypeID, usageUser, KeySuccessors)
 	registerKey(KeyPrequel, TypeIDSet, usageUser, KeySequel)
 	registerKey(KeyPublished, TypeTimestamp, usageProperty, "")
 	registerKey(KeyQuery, TypeEmpty, usageUser, "")
 	registerKey(KeyReadOnly, TypeWord, usageUser, "")
 	registerKey(KeySummary, TypeString, usageUser, "")
@@ -277,27 +273,40 @@
 
 // SetNonEmpty stores the given value under the given key, if the value is non-empty.
 // An empty value will delete the previous association.
 func (m *Meta) SetNonEmpty(key string, value Value) {
 	if value == "" {
-		delete(m.pairs, key) // TODO: key != KeyID
+		if key != KeyID {
+			delete(m.pairs, key)
+		}
 	} else {
 		m.Set(key, value.TrimSpace())
 	}
 }
+
+// Has returns true, if the given key is used in the metadata.
+func (m *Meta) Has(key string) bool {
+	if m != nil {
+		if _, found := m.pairs[key]; found || key == KeyID {
+			return true
+		}
+	}
+	return false
+}
 
 // Get retrieves the string value of a given key. The bool value signals,
 // whether there was a value stored or not.
 func (m *Meta) Get(key string) (Value, bool) {
-	if m == nil {
-		return "", false
-	}
-	if key == KeyID {
-		return Value(m.Zid.String()), true
-	}
-	value, ok := m.pairs[key]
-	return value, ok
+	if m != nil {
+		if value, found := m.pairs[key]; found {
+			return value, true
+		}
+		if key == KeyID {
+			return Value(m.Zid.String()), true
+		}
+	}
+	return "", false
 }
 
 // GetDefault retrieves the string value of the given key. If no value was
 // stored, the given default value is returned.
 func (m *Meta) GetDefault(key string, def Value) Value {

Index: domain/meta/type.go
==================================================================
--- domain/meta/type.go
+++ domain/meta/type.go
@@ -92,10 +92,12 @@
 	cachedTypedKeys = make(map[string]*DescriptionType)
 	mxTypedKey      sync.RWMutex
 	suffixTypes     = map[string]*DescriptionType{
 		"-date":       TypeTimestamp,
 		"-number":     TypeNumber,
+		"-ref":        TypeID,
+		"-refs":       TypeIDSet,
 		SuffixKeyRole: TypeWord,
 		"-time":       TypeTimestamp,
 		SuffixKeyURL:  TypeURL,
 		"-zettel":     TypeID,
 		"-zid":        TypeID,

Index: go.mod
==================================================================
--- go.mod
+++ go.mod
@@ -1,11 +1,11 @@
 module t73f.de/r/zsc
 
 go 1.24
 
 require (
-	t73f.de/r/sx v0.0.0-20250415161954-42ed9c4d6abc
-	t73f.de/r/sxwebs v0.0.0-20250415162443-110f49c5a1ae
-	t73f.de/r/webs v0.0.0-20250311182734-f263a38b32d5
-	t73f.de/r/zero v0.0.0-20250226205915-c4194684acb7
-	t73f.de/r/zsx v0.0.0-20250415162540-fc13b286b6ce
+	t73f.de/r/sx v0.0.0-20250707071435-95b82f7d24bb
+	t73f.de/r/sxwebs v0.0.0-20250806170342-a33d11a3e7c5
+	t73f.de/r/webs v0.0.0-20250723141744-5e8deae4d17b
+	t73f.de/r/zero v0.0.0-20250703105709-bb38976d4455
+	t73f.de/r/zsx v0.0.0-20250707071920-5e29047e4db7
 )

Index: go.sum
==================================================================
--- go.sum
+++ go.sum
@@ -1,10 +1,10 @@
-t73f.de/r/sx v0.0.0-20250415161954-42ed9c4d6abc h1:tlsP+47Rf8i9Zv1TqRnwfbQx3nN/F/92RkT6iCA6SVA=
-t73f.de/r/sx v0.0.0-20250415161954-42ed9c4d6abc/go.mod h1:hzg05uSCMk3D/DWaL0pdlowfL2aWQeGIfD1S04vV+Xg=
-t73f.de/r/sxwebs v0.0.0-20250415162443-110f49c5a1ae h1:K6nxN/bb0BCSiDffwNPGTF2uf5WcTdxcQXzByXNuJ7M=
-t73f.de/r/sxwebs v0.0.0-20250415162443-110f49c5a1ae/go.mod h1:0LQ9T1svSg9ADY/6vQLKNUu6LqpPi8FGr7fd2qDT5H8=
-t73f.de/r/webs v0.0.0-20250311182734-f263a38b32d5 h1:nnKfs/2i9n3S5VjbSj98odcwZKGcL96qPSIUATT/2P8=
-t73f.de/r/webs v0.0.0-20250311182734-f263a38b32d5/go.mod h1:zk92hSKB4iWyT290+163seNzu350TA9XLATC9kOldqo=
-t73f.de/r/zero v0.0.0-20250226205915-c4194684acb7 h1:OuzHSfniY8UzLmo5zp1w23Kd9h7x9CSXP2jQ+kppeqU=
-t73f.de/r/zero v0.0.0-20250226205915-c4194684acb7/go.mod h1:T1vFcHoymUQcr7+vENBkS1yryZRZ3YB8uRtnMy8yRBA=
-t73f.de/r/zsx v0.0.0-20250415162540-fc13b286b6ce h1:R9rtg4ecx4YYixsMmsh+wdcqLdY9GxoC5HZ9mMS33to=
-t73f.de/r/zsx v0.0.0-20250415162540-fc13b286b6ce/go.mod h1:tXOlmsQBoY4mY7Plu0LCCMZNSJZJbng98fFarZXAWvM=
+t73f.de/r/sx v0.0.0-20250707071435-95b82f7d24bb h1:cYvTOpaJinh/EPB7i8nx7PtT7hniuSP+NZr74P9U+fE=
+t73f.de/r/sx v0.0.0-20250707071435-95b82f7d24bb/go.mod h1:uglbFdRHlcpQVVyCNh4Fd7jbKo8alGBCjRp0aZv8IIg=
+t73f.de/r/sxwebs v0.0.0-20250806170342-a33d11a3e7c5 h1:7r1SP0h9QySrRDz9qYWZ/xuEEyJi3XYHtxRIoW2BhoM=
+t73f.de/r/sxwebs v0.0.0-20250806170342-a33d11a3e7c5/go.mod h1:CvFDV0czGR0qFVdTYrdy8WIIu5OvA3tFD/td1mim/lA=
+t73f.de/r/webs v0.0.0-20250723141744-5e8deae4d17b h1:LmJ0STcaUyGgH2jVa/nKifmJedl0njdnMwPxqX6zLQg=
+t73f.de/r/webs v0.0.0-20250723141744-5e8deae4d17b/go.mod h1:b8/5E5Pe6WSWqh+T+sxLO5ZLiGVkuL5tgh86kx2OAIg=
+t73f.de/r/zero v0.0.0-20250703105709-bb38976d4455 h1:TFRPPexX2WrwuF03hC+Be2ONx2bPzMMBlNDn0rk88eI=
+t73f.de/r/zero v0.0.0-20250703105709-bb38976d4455/go.mod h1:Ovx7CYsjz45BNuIEMGZfqA7NdQxERydJqUGnOBoQaXQ=
+t73f.de/r/zsx v0.0.0-20250707071920-5e29047e4db7 h1:ERxpb1Hqln+NXoZDK6sqjmX3BzeoLO+O64f4bK0B6dk=
+t73f.de/r/zsx v0.0.0-20250707071920-5e29047e4db7/go.mod h1:64/AjQ1GnEBoBhXI1D0bDMGDj7JCbtZUTT3WoA7kS0s=

Index: sexp/sexp.go
==================================================================
--- sexp/sexp.go
+++ sexp/sexp.go
@@ -19,10 +19,11 @@
 	"errors"
 	"fmt"
 	"sort"
 
 	"t73f.de/r/sx"
+	"t73f.de/r/sx/sxbuiltins"
 	"t73f.de/r/zsc/api"
 )
 
 // EncodeZettel transforms zettel data into a sx object.
 func EncodeZettel(zettel api.ZettelData) sx.Object {
@@ -80,11 +81,11 @@
 }
 
 // EncodeMetaRights translates metadata/rights into a sx object.
 func EncodeMetaRights(mr api.MetaRights) *sx.Pair {
 	return sx.MakeList(
-		sx.SymbolList,
+		sx.MakeSymbol(sxbuiltins.List.Name),
 		meta2sz(mr.Meta),
 		sx.MakeList(sx.MakeSymbol("rights"), sx.Int64(int64(mr.Rights))),
 	)
 }
 

Index: shtml/shtml.go
==================================================================
--- shtml/shtml.go
+++ shtml/shtml.go
@@ -70,14 +70,11 @@
 		key := keys[i]
 		if key != zsx.DefaultAttribute && isValidName(key) {
 			plist = plist.Cons(sx.Cons(sx.MakeSymbol(key), sx.MakeString(a[key])))
 		}
 	}
-	if plist == nil {
-		return nil
-	}
-	return plist.Cons(sxhtml.SymAttr)
+	return plist
 }
 
 // Evaluate a metadata s-expression into a list of HTML s-expressions.
 func (ev *Evaluator) Evaluate(lst *sx.Pair, env *Environment) (*sx.Pair, error) {
 	result := ev.Eval(lst, env)
@@ -123,26 +120,24 @@
 	}
 
 	var result sx.ListBuilder
 	result.AddN(
 		SymOL,
-		sx.Nil().Cons(sx.Cons(SymAttrClass, sx.MakeString("zs-endnotes"))).Cons(sxhtml.SymAttr),
+		sx.Nil().Cons(sx.Cons(SymAttrClass, sx.MakeString("zs-endnotes"))),
 	)
 	for i, fni := range env.endnotes {
 		noteNum := strconv.Itoa(i + 1)
 		attrs := fni.attrs.Cons(sx.Cons(SymAttrClass, sx.MakeString("zs-endnote"))).
 			Cons(sx.Cons(SymAttrValue, sx.MakeString(noteNum))).
 			Cons(sx.Cons(SymAttrID, sx.MakeString("fn:"+fni.noteID))).
-			Cons(sx.Cons(SymAttrRole, sx.MakeString("doc-endnote"))).
-			Cons(sxhtml.SymAttr)
+			Cons(sx.Cons(SymAttrRole, sx.MakeString("doc-endnote")))
 
 		backref := sx.Nil().Cons(sx.MakeString("\u21a9\ufe0e")).
 			Cons(sx.Nil().
 				Cons(sx.Cons(SymAttrClass, sx.MakeString("zs-endnote-backref"))).
 				Cons(sx.Cons(SymAttrHref, sx.MakeString("#fnref:"+fni.noteID))).
-				Cons(sx.Cons(SymAttrRole, sx.MakeString("doc-backlink"))).
-				Cons(sxhtml.SymAttr)).
+				Cons(sx.Cons(SymAttrRole, sx.MakeString("doc-backlink")))).
 			Cons(SymA)
 
 		var li sx.ListBuilder
 		li.AddN(SymLI, attrs)
 		li.ExtendBang(fni.noteHx)
@@ -642,14 +637,13 @@
 		noteID := ev.unique + noteNum
 		env.endnotes = append(env.endnotes, endnoteInfo{
 			noteID: noteID, noteAST: args[1:], noteHx: nil, attrs: attrPlist})
 		hrefAttr := sx.Nil().Cons(sx.Cons(SymAttrRole, sx.MakeString("doc-noteref"))).
 			Cons(sx.Cons(SymAttrHref, sx.MakeString("#fn:"+noteID))).
-			Cons(sx.Cons(SymAttrClass, sx.MakeString("zs-noteref"))).
-			Cons(sxhtml.SymAttr)
+			Cons(sx.Cons(SymAttrClass, sx.MakeString("zs-noteref")))
 		href := sx.Nil().Cons(sx.MakeString(noteNum)).Cons(hrefAttr).Cons(SymA)
-		supAttr := sx.Nil().Cons(sx.Cons(SymAttrID, sx.MakeString("fnref:"+noteID))).Cons(sxhtml.SymAttr)
+		supAttr := sx.Nil().Cons(sx.Cons(SymAttrID, sx.MakeString("fnref:"+noteID)))
 		return sx.Nil().Cons(href).Cons(supAttr).Cons(symSUP)
 	})
 
 	ev.bind(zsx.SymFormatDelete, 1, ev.makeFormatFn(symDEL))
 	ev.bind(zsx.SymFormatEmph, 1, ev.makeFormatFn(symEM))

Index: sz/zmk/zmk_test.go
==================================================================
--- sz/zmk/zmk_test.go
+++ sz/zmk/zmk_test.go
@@ -66,12 +66,12 @@
 	}
 }
 
 type astWalker struct{}
 
-func (astWalker) VisitBefore(node *sx.Pair, env *sx.Pair) (sx.Object, bool) { return sx.Nil(), false }
-func (astWalker) VisitAfter(node *sx.Pair, env *sx.Pair) sx.Object          { return node }
+func (astWalker) VisitBefore(*sx.Pair, *sx.Pair) (sx.Object, bool) { return sx.Nil(), false }
+func (astWalker) VisitAfter(node *sx.Pair, _ *sx.Pair) sx.Object   { return node }
 
 func TestEdges(t *testing.T) {
 	t.Parallel()
 	checkTcs(t, TestCases{
 		{"\"\"\"\n; \n0{{0}}{0}\n\"\"\"", "(BLOCK (REGION-VERSE () ((DESCRIPTION () ()) (PARA (TEXT \"0\") (EMBED ((\"0\" . \"\")) (HOSTED \"0\") \"\")))))"},