Let a declared datatype win over a column read, count a repeat as written, refuse null alone on stdin and a second leading slash, and hand NewTemplate stdin as sent

internal/grammar/template.go

@@ -16,12 +16,23 @@ func IsTemplate(arg string) (bool, error) {
 	if strings.HasPrefix(strings.TrimSpace(arg), "[") {
 		return false, fmt.Errorf(`%q starts with "[" and is not valid JSON, so it names no template; a path starts with a name`, arg)
 	}
-	_, err := SplitPath(CallerPath(arg))
+	path, err := CallerPath(arg)
+	if err != nil {
+		return false, err
+	}
+	_, err = SplitPath(path)
 	return false, err
 }
 
-// CallerPath is a path as a caller passed it, without the slashes it may start with.
-func CallerPath(path string) string { return strings.TrimLeft(path, "/") }
+// CallerPath is a path as a caller passed it, without the one / it may start with, as a
+// reference does.
+func CallerPath(path string) (string, error) {
+	rest := strings.TrimPrefix(path, "/")
+	if strings.HasPrefix(rest, "/") {
+		return "", fmt.Errorf("path %s starts with more than one /; write /%s", path, strings.TrimLeft(rest, "/"))
+	}
+	return rest, nil
+}
 
 func isJSONStart(arg string) bool {
 	return strings.HasPrefix(arg, "[") || strings.HasPrefix(arg, `"`)