Answer architecture round 3, stability round 1 and product-owner round 1: CalcName and proven.OfCalc for one meaning per name, a lazy load placed where its lookup found it, DataType's constants typed under it, and cover printing the module's total

internal/builtinfunc/builtinfunc.go

@@ -43,9 +43,9 @@ 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"
+// CalcName is the builtin whose operands the engine checks and proves, since only the engine
+// holds their nodes.
+const CalcName = "calc"
 
 // rng is the randomness a builtin sample draws from, which *drawstate.State satisfies.
 type rng interface {
@@ -115,7 +115,7 @@ func numberCalls(text bool) string {
 		}
 	}
 	slices.Sort(calls)
-	return strings.Join(calls, ", ") + " or {calc()}"
+	return strings.Join(calls, ", ") + " or {" + CalcName + "()}"
 }
 
 func plural(n int) string {

internal/builtinfunc/builtins.go

@@ -64,9 +64,9 @@ var builtins = withTransforms(map[string]builtin{
 		cc := a[0]
 		return func(s *drawstate.State, _ string, _ []string) string { return iban(s, cc) }
 	}},
-	"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},
+	"date":   {arity: -1, checkArgs: dateArgs, prep: datePrep},
+	"time":   {arity: -1, checkArgs: timeArg, prep: timePrep},
+	CalcName: {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.