Answer architecture round 2: guard Noun's range, name calc once as builtinfunc.Calc, and place readTableOptions in item 46

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

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.

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]
+}