diff --git a/builtins.go b/builtins.go
index a26405a..bf1889b 100644
--- a/builtins.go
+++ b/builtins.go
@@ -24,13 +24,11 @@ const (
 // clock. Add a builtin only for what data can't express: a random v4 UUID and a
 // 24-hex ObjectID both ship as data, so the uuid builtin is v7.
 var builtins = map[string]builtin{
-	"luhn": {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string {
-		return string(rune('0' + luhnCheck(e)))
-	}},
-	"mod11": {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string { return mod11Check(e) }},
-	"ean":   {arity: 0, call: func(_ *session, e string, _ map[string]node, _ []string) string { return eanCheck(e) }},
-	"uuid":  {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return uuidV7(s) }},
-	"ulid":  {arity: 0, call: func(s *session, _ string, _ map[string]node, _ []string) string { return ulid(s) }},
+	"luhn":  {arity: 0, prep: derive(func(e string) string { return string(rune('0' + luhnCheck(e))) })},
+	"mod11": {arity: 0, prep: derive(mod11Check)},
+	"ean":   {arity: 0, prep: derive(eanCheck)},
+	"uuid":  {arity: 0, prep: generate(uuidV7)},
+	"ulid":  {arity: 0, prep: generate(ulid)},
 	"nanoid": {arity: 1, check: posIntArg, prep: func(a []string) callFn {
 		n := atoi(a[0])
 		return func(s *session, _ string, _ map[string]node) string { return nanoid(s, n) }
@@ -57,7 +55,10 @@ var builtins = map[string]builtin{
 			return strconv.FormatFloat(lo+s.Float64()*(hi-lo), 'f', dp, 64)
 		}
 	}},
-	"iban": {arity: 1, check: ibanArg, call: func(s *session, _ string, _ map[string]node, a []string) string { return iban(s, a[0]) }},
+	"iban": {arity: 1, check: ibanArg, prep: func(a []string) callFn {
+		cc := a[0]
+		return func(s *session, _ string, _ map[string]node) string { return iban(s, cc) }
+	}},
 	// calc is the one builtin that reads the sibling fields (to render its operands);
 	// every other ignores them. 1 or 2 args: the expression and an optional decimals.
 	"calc": {arity: -1, check: checkCalc, prep: calcPrep},
@@ -75,6 +76,21 @@ var builtins = map[string]builtin{
 	}},
 }
 
+// derive and generate are the two argument-free builtin shapes the README names: a
+// derivation reads the output emitted so far, a generator reads only the rng. Each
+// lifts that one function into the prep every registry entry supplies.
+func derive(f func(emitted string) string) func([]string) callFn {
+	return func([]string) callFn {
+		return func(_ *session, emitted string, _ map[string]node) string { return f(emitted) }
+	}
+}
+
+func generate(f func(rng) string) func([]string) callFn {
+	return func([]string) callFn {
+		return func(s *session, _ string, _ map[string]node) string { return f(s) }
+	}
+}
+
 const hexDigits = "0123456789abcdef"
 
 // atoi parses an arg already validated by a builtin's check, so it cannot fail.
diff --git a/builtins_test.go b/builtins_test.go
index 3fe6e6b..0a41f3a 100644
--- a/builtins_test.go
+++ b/builtins_test.go
@@ -181,18 +181,14 @@ func ibanValid(s string) bool {
 	return rem == 1
 }
 
-// TestRegistryShapes pins the builtin contract bind relies on: exactly one of prep
-// or call, and args parsed at compile only behind a check. A registry entry with
-// neither would compile and then panic inside a render.
+// TestRegistryShapes pins the builtin contract compileOps relies on: every entry
+// supplies prep, and args parsed at compile only behind a check.
 func TestRegistryShapes(t *testing.T) {
 	for name, b := range builtins {
-		switch {
-		case b.prep == nil && b.call == nil:
-			t.Errorf("builtin %q has neither prep nor call: bind would return a nil-func closure", name)
-		case b.prep != nil && b.call != nil:
-			t.Errorf("builtin %q has both prep and call; call is dead", name)
+		if b.prep == nil {
+			t.Errorf("builtin %q has no prep: compileOps would call a nil func", name)
 		}
-		if b.prep != nil && b.arity != 0 && b.check == nil {
+		if b.arity != 0 && b.check == nil {
 			t.Errorf("builtin %q parses args in prep with no check", name)
 		}
 	}
diff --git a/template.go b/template.go
index 1d660a2..06c2036 100644
--- a/template.go
+++ b/template.go
@@ -67,15 +67,12 @@ func eachToken(format string, fn func(ftoken) error) error {
 // seq is the one exception: it advances per-session counter state, which is itself
 // deterministic (1, 2, 3 …). arity is the exact arg count, or -1 for variadic
 // (then check does all the validation). The optional check validates args at
-// compile time (their values, beyond the count). A builtin supplies exactly one of
-// prep (args parsed once, at compile) or call (args parsed per render). The registry
-// lives in builtins.go.
+// compile time (their values, beyond the count). The registry lives in builtins.go.
 type builtin struct {
 	arity int
 	// prep parses validated args once, at compile time, into the closure expand calls.
 	prep  func(args []string) callFn
 	check func(fields map[string]node, args []string) error
-	call  func(s *session, emitted string, fields map[string]node, args []string) string
 }
 
 // funcCall splits a "{token}" body shaped name(args) into its parts; ok is false
@@ -327,7 +324,7 @@ func compileOps(format string) ([]op, int, map[string]string) {
 		case 'b':
 			flush()
 			if name, args, ok := funcCall(t.body); ok {
-				ops = append(ops, op{kind: 'b', call: builtins[name].bind(args)})
+				ops = append(ops, op{kind: 'b', call: builtins[name].prep(args)})
 			} else {
 				arms := splitArms(t.body)
 				for _, a := range arms {
@@ -348,14 +345,3 @@ func compileOps(format string) ([]op, int, map[string]string) {
 	flush()
 	return ops, grow, bound
 }
-
-// bind returns the closure for one call site, parsing args once via prep if present.
-func (b builtin) bind(args []string) callFn {
-	if b.prep != nil {
-		return b.prep(args)
-	}
-	call := b.call
-	return func(s *session, emitted string, fields map[string]node) string {
-		return call(s, emitted, fields, args)
-	}
-}