Fix handling of empty propositions on the command line

cli.lisp

@@ -37,13 +37,13 @@ functions involved in evaluating and typesetting."
              (parse-proposition-string prop-str
                                        :implicit-and implicit-and
                                        :multi-char-names multi-char-names))
+        when parsed-exp
           collect parsed-exp into exps
         do (dolist (var parsed-vars)
-             (unless (member var vars :test 'equal)
-               (setq vars (nconc vars (list var)))))
+             (pushnew var vars :test 'equal))
         finally
            (let ((table (create-combined-truth-table
-                         exps vars
+                         exps (nreverse vars)
                          :include-intermediate include-intermediate
                          :include-vars include-vars)))
              (return (typeset-table-to-format table format