diff --git a/README.md b/README.md
index 08269f1..66dbb2d 100644
--- a/README.md
+++ b/README.md
@@ -413,7 +413,7 @@ datatype cannot hold is a load error naming it:
 
 ```text
 order.id: datatype integer: {digits(3)} prints text, not an integer
-order.id: datatype integer: "1{digits(2)}" is not one value; write one literal or one {int()}, {float()}, {seq()} or {calc()}, or read one
+order.id: datatype integer: "1{digits(2)}" is not one value; write one literal or one {float()}, {int()}, {seq()} or {calc()}, or read one
 ```
 
 A column of one reference alone to another record's column — `"score": "{/src.score}"`
diff --git a/builtins.go b/builtins.go
index c6d6c0d..d960701 100644
--- a/builtins.go
+++ b/builtins.go
@@ -22,7 +22,7 @@ const (
 // emitted so far in the current expansion (place them after their payload);
 // samples read only the rng. A time-based id (uuid v7, ulid) draws its timestamp
 // from the rng, not the wall clock, so seeded output stays reproducible.
-var builtins = map[string]builtin{
+var builtins = withTransforms(map[string]builtin{
 	"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)},
@@ -30,8 +30,8 @@ var builtins = map[string]builtin{
 	"ulid":   {arity: 0, prep: sample(ulid)},
 	"nanoid": {arity: 1, check: posIntArg, prep: chars(nanoidAlphabet)},
 	"hex":    {arity: 1, check: posIntArg, prep: chars(hexDigits)},
-	"digits": {arity: 1, check: posIntArg, prep: chars("0123456789"), number: func(token string, a []string) proven {
-		return printing(token, DataTypeString, bounded(0, math.Pow(10, float64(atoi(a[0])))-1, true))
+	"digits": {arity: 1, check: posIntArg, prep: chars("0123456789"), prints: DataTypeString, number: func(token string, prints DataType, a []string) proven {
+		return printing(token, prints, bounded(0, math.Pow(10, float64(atoi(a[0])))-1, true))
 	}},
 	"upper": {arity: 1, check: posIntArg, prep: chars("ABCDEFGHIJKLMNOPQRSTUVWXYZ")},
 	"lower": {arity: 1, check: posIntArg, prep: chars("abcdefghijklmnopqrstuvwxyz")},
@@ -44,27 +44,24 @@ var builtins = map[string]builtin{
 	"int": {arity: 2, check: intRangeArgs, prep: func(a []string) callFn {
 		lo, span := atoi(a[0]), atoi(a[1])-atoi(a[0])+1
 		return func(s *session, _ string, _ []string) string { return strconv.Itoa(lo + s.IntN(span)) }
-	}, number: func(token string, a []string) proven {
-		return printing(token, DataTypeInteger, bounded(float64(atoi(a[0])), float64(atoi(a[1])), true))
+	}, prints: DataTypeInteger, number: func(token string, prints DataType, a []string) proven {
+		return printing(token, prints, bounded(float64(atoi(a[0])), float64(atoi(a[1])), true))
 	}},
 	"float": {arity: 3, check: floatArgs, prep: func(a []string) callFn {
 		lo, hi, dp := atof(a[0]), atof(a[1]), atoi(a[2])
 		return func(s *session, _ string, _ []string) string {
 			return formatFloat(lo+s.Float64()*(hi-lo), dp)
 		}
-	}, number: func(token string, a []string) proven {
+	}, prints: DataTypeNumber, number: func(token string, _ DataType, a []string) proven {
 		return printedNumber(token, bounded(atof(a[0]), atof(a[1]), false), atoi(a[2]))
 	}},
 	"iban": {arity: 1, check: ibanArg, prep: func(a []string) callFn {
 		cc := a[0]
 		return func(s *session, _ string, _ []string) string { return iban(s, cc) }
 	}},
-	"date":      {arity: -1, check: dateArgs, prep: datePrep},
-	"time":      {arity: -1, check: timeArg, prep: timePrep},
-	"calc":      {arity: -1, check: checkCalc, prep: calcPrep, operands: calcOperands},
-	"lowercase": {arity: 1, check: transformArg, prep: transformPrep(strings.ToLower), operands: transformOperand},
-	"uppercase": {arity: 1, check: transformArg, prep: transformPrep(strings.ToUpper), operands: transformOperand},
-	"ascii":     {arity: 1, check: transformArg, prep: transformPrep(asciiFold), operands: transformOperand},
+	"date": {arity: -1, check: dateArgs, prep: datePrep},
+	"time": {arity: -1, check: timeArg, prep: timePrep},
+	"calc": {arity: -1, check: checkCalc, prep: calcPrep, operands: calcOperands},
 	// seq is the one stateful builtin: a per-session counter from 1, advancing on
 	// each call. An optional name selects an independent counter; no name uses the
 	// default one. Deterministic by construction, so seeded output stays stable.
@@ -76,10 +73,10 @@ var builtins = map[string]builtin{
 		return func(s *session, _ string, _ []string) string {
 			return strconv.FormatUint(s.next(key), 10)
 		}
-	}, number: func(token string, _ []string) proven {
-		return printing(token, DataTypeInteger, bounded(1, math.MaxInt64, true))
+	}, prints: DataTypeInteger, number: func(token string, prints DataType, _ []string) proven {
+		return printing(token, prints, bounded(1, math.MaxInt64, true))
 	}},
-}
+})
 
 // derive and sample are the two argument-free builtin shapes: a derivation reads
 // the output emitted so far, a sample reads only the rng. chars is the shape of a
diff --git a/template.go b/template.go
index b5f8a45..d23dd0b 100644
--- a/template.go
+++ b/template.go
@@ -119,8 +119,9 @@ type builtin struct {
 	// for a builtin that reads none.
 	operands func(args []string) []string
 	// number proves what a call prints, token its body: the bounds of its number and the
-	// datatype of its text; nil for a builtin whose text is no number.
-	number func(token string, args []string) proven
+	// datatype of its text, prints or one inside it; nil for a builtin whose text is no number.
+	prints DataType
+	number func(token string, prints DataType, args []string) proven
 }
 
 // funcCall splits a "{token}" body shaped name(args) into its parts; ok is false
diff --git a/transform.go b/transform.go
index 2b9fecd..0515889 100644
--- a/transform.go
+++ b/transform.go
@@ -14,6 +14,13 @@ var transforms = map[string]func(string) string{
 	"uppercase": strings.ToUpper,
 }
 
+func withTransforms(builtins map[string]builtin) map[string]builtin {
+	for name, fn := range transforms {
+		builtins[name] = builtin{arity: 1, check: transformArg, prep: transformPrep(fn), operands: transformOperand}
+	}
+	return builtins
+}
+
 // unwrapTransform peels nested transform calls off an operand arg, returning the
 // field it finally names and the transforms to apply, innermost last.
 func unwrapTransform(arg string) (leaf string, chain []func(string) string, err error) {
diff --git a/value.go b/value.go
index 9b310c6..d217fd0 100644
--- a/value.go
+++ b/value.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"math"
 	"regexp"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -148,8 +149,8 @@ func (p *valueProof) template(t *template) proven {
 	case t.fixed:
 		return literalValue(t.lit)
 	case len(t.ops) != 1:
-		v := unproven(notOneValue(t.format, "{int()}, {float()}, {seq()} or {calc()}"))
-		v.notOperand = notOneValue(t.format, "{int()}, {float()}, {seq()}, {digits()} or {calc()}")
+		v := unproven(notOneValue(t.format, numberCalls(false)))
+		v.notOperand = notOneValue(t.format, numberCalls(true))
 		return v
 	}
 	o := t.ops[0]
@@ -164,7 +165,7 @@ func (p *valueProof) template(t *template) proven {
 	case name == "calc":
 		return p.calc(t, body, args)
 	case builtins[name].number != nil:
-		return builtins[name].number(body, args)
+		return builtins[name].number(body, builtins[name].prints, args)
 	case isTransform:
 		return unproven(fmt.Sprintf("{%s} rewrites text rather than printing a value; write the values it would print", body))
 	}
@@ -287,6 +288,19 @@ func printing(token string, prints DataType, v proven) proven {
 	return v
 }
 
+// numberCalls lists the builtins printing a number, or with text also those whose text
+// is one.
+func numberCalls(text bool) string {
+	var calls []string
+	for name, b := range builtins {
+		if b.number != nil && (text || b.prints != DataTypeString) {
+			calls = append(calls, "{"+name+"()}")
+		}
+	}
+	slices.Sort(calls)
+	return strings.Join(calls, ", ") + " or {calc()}"
+}
+
 func notOneValue(format, calls string) string {
 	return fmt.Sprintf("%q is not one value; write one literal or one %s, or read one", format, calls)
 }