Name digits among an operand's spellings, and document number by what its text is

datatype_test.go

@@ -65,6 +65,7 @@ func TestDatatypeRejectsAValueItsTypeRejects(t *testing.T) {
 		{"a divisor that can be zero", `{"format":"{calc(a / b)}","a":"{int(1,9)}","b":"{int(0,9)}","datatype":"number"}`, "divides by b, which is not proven nonzero"},
 		{"an overflow", `{"format":"{calc(a * a)}","a":"{digits(200)}","datatype":"number"}`, "is not proven within 1e300"},
 		{"a division in an integer column", `{"format":"{calc(a / b)}","a":"{int(1,9)}","b":"{int(1,9)}","datatype":"integer"}`, "prints a number, not an integer"},
+		{"a composed operand", `{"format":"{calc(n * 2)}","n":"{int(1,99)}.{digits(2)}","datatype":"number"}`, "{seq()}, {digits()} or {calc()}"},
 	} {
 		row := `{"format":"","col":` + c.column + `}`
 		files := map[string]string{"row": row}

template.go

@@ -73,7 +73,7 @@ type builtin struct {
 	// for a builtin that reads none.
 	operands func(args []string) []string
 	// number bounds the number a call prints and names the datatype its text is; nil
-	// for a builtin that prints text.
+	// for a builtin whose text is no number.
 	number func(args []string) (proven, DataType)
 }
 

value.go

@@ -92,7 +92,9 @@ func (p *valueProof) template(t *template) proven {
 	case t.fixed:
 		return literalValue(t.lit)
 	case len(t.ops) != 1:
-		return unproven(fmt.Sprintf("%q is not one value; write one literal or one {int()}, {float()}, {seq()} or {calc()}, or read one", t.format))
+		v := unproven(notOneValue(t.format, "{int()}, {float()}, {seq()} or {calc()}"))
+		v.notNumber = notOneValue(t.format, "{int()}, {float()}, {seq()}, {digits()} or {calc()}")
+		return v
 	}
 	body := t.format[1 : len(t.format)-1]
 	name, args, isFunc := funcCall(body)
@@ -227,6 +229,10 @@ func printing(token string, prints DataType, v proven) proven {
 	return v
 }
 
+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)
+}
+
 // unproven is a render no datatype and no calc can take, for why.
 func unproven(why string) proven {
 	v := proven{notNumber: why}