Check a typed column's value, not its text: one literal, value call, calc or read, bounded by its arguments

template.go

@@ -72,9 +72,9 @@ type builtin struct {
 	// operands names the fields the call reads, which expand renders for it; nil
 	// for a builtin that reads none.
 	operands func(args []string) []string
-	// emits is the text a call can print, for the datatype check; nil for calc and the
-	// transforms, whose text the check derives from what they read.
-	emits func(args []string) textShape
+	// number bounds the number a call prints and names the datatype its text is; nil
+	// for a builtin that prints text.
+	number func(args []string) (proven, DataType)
 }
 
 // funcCall splits a "{token}" body shaped name(args) into its parts; ok is false