diff --git a/README.md b/README.md
index b565a2b..d66a341 100644
--- a/README.md
+++ b/README.md
@@ -365,9 +365,10 @@ the reference sits, including in a field the format renders.
 
 A `{calc()}` operand is held on its own terms too, so the same fence guards it:
 `{..cat.net} x 2 = {calc(net * 2, 2)}` names one field two ways and is a load
-error, with no path token anywhere. What an operand's draw fixes is the value it
-renders, so a reference reaching *through* it is caught as well, while a sibling
-its format never renders is free — that sibling is no part of the value.
+error, with no path token anywhere. A calc renders its operand whole, so what the
+hold pins is that one field — another route conflicts only by naming it. Two
+names that merely draw from one source are two draws, as `{word} {word}` is, so
+two dice over one `{..die}` are fine.
 
 ```text
 token {p} renders a level that {p.first} reads a path into; name the fields you want instead
diff --git a/builtins.go b/builtins.go
index 943fcd3..4249a32 100644
--- a/builtins.go
+++ b/builtins.go
@@ -48,9 +48,7 @@ var builtins = map[string]builtin{
 		return func(s *session, _ string, _ []string) string { return strconv.Itoa(lo + s.IntN(span)) }
 	}},
 	"float": {arity: 3, check: floatArgs, prep: func(a []string) callFn {
-		lo, _ := strconv.ParseFloat(a[0], 64)
-		hi, _ := strconv.ParseFloat(a[1], 64)
-		dp := atoi(a[2])
+		lo, hi, dp := atof(a[0]), atof(a[1]), atoi(a[2])
 		return func(s *session, _ string, _ []string) string {
 			return strconv.FormatFloat(lo+s.Float64()*(hi-lo), 'f', dp, 64)
 		}
@@ -104,6 +102,15 @@ func atoi(s string) int {
 	return n
 }
 
+// atof is atoi for a float arg, and reports an unvalidated one the same way.
+func atof(s string) float64 {
+	f, err := strconv.ParseFloat(s, 64)
+	if err != nil {
+		panic(fmt.Sprintf("fakes: builtin arg %q reached prep unvalidated: %v", s, err))
+	}
+	return f
+}
+
 func randBytes(r rng, n int) []byte {
 	b := make([]byte, n)
 	for i := range b {
diff --git a/render.go b/render.go
index e5b2952..ee60f9c 100644
--- a/render.go
+++ b/render.go
@@ -156,7 +156,8 @@ func expand(s *session, t *template) string {
 		case 'f':
 			b.WriteString(readField(s, t, held, o.arms[s.IntN(len(o.arms))]))
 		case 'b':
-			// Read in the order calcVars fixed, which is what op.operands holds.
+			// Read before the call, so the value a calc computes is the value the
+			// format showed. calcVars fixed the order op.operands holds.
 			var operands []string
 			if len(o.operands) > 0 {
 				operands = make([]string, len(o.operands))