diff --git a/calc.go b/calc.go
index 1cf8ff5..818349b 100644
--- a/calc.go
+++ b/calc.go
@@ -26,7 +26,7 @@ func checkCalcFields(args []string, fields map[string]node) error {
 // checkCalcNames holds each calc of t reading a name to the checks checkCalcFields makes of a field.
 func checkCalcNames(path string, t *template) error {
 	for _, o := range t.compiled.ops {
-		if o.Fn != "calc" || !slices.ContainsFunc(o.operands, func(a arm) bool { return a.kind == namedRead }) {
+		if o.Fn != builtinfunc.Calc || !slices.ContainsFunc(o.operands, func(a arm) bool { return a.kind == namedRead }) {
 			continue
 		}
 		if err := checkOperands(o.Args[0], builtinfunc.ParsedCalc(o.Args[0]), operandNodes(o)); err != nil {
diff --git a/format.go b/format.go
index b090de6..65de46e 100644
--- a/format.go
+++ b/format.go
@@ -12,7 +12,7 @@ import (
 // operands fields holds.
 func checkFunc(tok grammar.Token, fields map[string]node) error {
 	err := builtinfunc.Check(tok.Fn, tok.Args)
-	if err == nil && tok.Fn == "calc" {
+	if err == nil && tok.Fn == builtinfunc.Calc {
 		err = checkCalcFields(tok.Args, fields)
 	}
 	if err != nil {
diff --git a/internal/builtinfunc/builtinfunc.go b/internal/builtinfunc/builtinfunc.go
index ed0c716..b6baf16 100644
--- a/internal/builtinfunc/builtinfunc.go
+++ b/internal/builtinfunc/builtinfunc.go
@@ -43,6 +43,10 @@ type builtin struct {
 	prints datatype.DataType
 }
 
+// Calc is the name of the builtin whose operands the caller checks and proves, as nodes only
+// it holds.
+const Calc = "calc"
+
 // rng is the randomness a builtin sample draws from, which *drawstate.State satisfies.
 type rng interface {
 	IntN(n int) int
diff --git a/internal/builtinfunc/builtins.go b/internal/builtinfunc/builtins.go
index 83f3443..8696650 100644
--- a/internal/builtinfunc/builtins.go
+++ b/internal/builtinfunc/builtins.go
@@ -66,7 +66,7 @@ var builtins = withTransforms(map[string]builtin{
 	}},
 	"date": {arity: -1, checkArgs: dateArgs, prep: datePrep},
 	"time": {arity: -1, checkArgs: timeArg, prep: timePrep},
-	"calc": {arity: -1, checkArgs: checkCalc, prep: calcPrep, operands: calcOperands, noRefOperands: true},
+	Calc:   {arity: -1, checkArgs: checkCalc, prep: calcPrep, operands: calcOperands, noRefOperands: true},
 	// seq is the one stateful builtin: a per-generator 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.
diff --git a/internal/datatype/datatype.go b/internal/datatype/datatype.go
index aa90ef5..f8b9200 100644
--- a/internal/datatype/datatype.go
+++ b/internal/datatype/datatype.go
@@ -31,4 +31,9 @@ func (d DataType) String() string {
 }
 
 // Noun is the datatype as an error names what a value is.
-func Noun(d DataType) string { return nouns[d] }
+func Noun(d DataType) string {
+	if d < 0 || d >= Count {
+		return d.String()
+	}
+	return nouns[d]
+}
diff --git a/todo.md b/todo.md
index 2d0344e..9e98ad7 100644
--- a/todo.md
+++ b/todo.md
@@ -380,7 +380,7 @@ Require the path step to reach a sibling category.
 
 ### 46. Report the same error every load for a table with two bad options, and for a folder with two unnamed rows files.
 
-`readTableOptions` and `walkDir` in `internal/datafiles` return on the first in Go's map order.
+`readTableOptions` in `table.go` and `walkDir` in `internal/datafiles` return on the first in Go's map order.
 
 ### 107. Move the template engine into `internal/`, leaving the root the public API and its options.
 
diff --git a/valueproof.go b/valueproof.go
index 7bc59cd..40bf2b3 100644
--- a/valueproof.go
+++ b/valueproof.go
@@ -173,7 +173,7 @@ func (p *valueProof) proveTemplate(t *template) proven.Value {
 			leaves = append(leaves, a.leaves...)
 		}
 		return p.proveUnion(leaves)
-	case name == "calc":
+	case name == builtinfunc.Calc:
 		return p.proveCalc(o)
 	case builtinfunc.IsTransform(name):
 		return proven.Unproven(fmt.Sprintf("{%s} rewrites text rather than printing a value; write the values it would print", body))