Answer architecture round 1: calls carry no operands in the grammar, ParseCalc returns its operands, one empty-segment check, sorted token kinds, and file items 110 and 111

path.go

@@ -90,11 +90,13 @@ func (t *table) route(tail []string, descended bool) (tableRoute, error) {
 // stepUp proves a ".." after a row of t can step up: t has a parent, rest names it, and no
 // selector follows, since a row selected below the parent row could lie outside it.
 func (t *table) stepUp(rest []string) error {
-	switch {
-	case t.parentT == nil:
+	if t.parentT == nil {
 		return fmt.Errorf(`%s has no parent table for ".." to step up to`, t.segment)
-	case len(rest) > 0 && rest[0] == "":
-		return fmt.Errorf("path has an empty segment")
+	}
+	if err := grammar.CheckSegments(rest); err != nil {
+		return err
+	}
+	switch {
 	case len(rest) == 0 || rest[0] != t.parentT.segment:
 		return fmt.Errorf(`".." steps up from %s to its parent table, so name that next: %s`, t.segment, t.climbTo(rest))
 	case grammar.HasSelector(rest):