Add is_set as to rules.yml

Signed-off-by: ClemensLinnhoff <clemens.linnhoff@persival.de>

Author: ClemensLinnhoff

rules.yml

@@ -9,4 +9,5 @@ refers_to: '^[ ]\b(refers_to)\b'
 is_iso_country_code: '^[ ]\b(is_iso_country_code)\b'
 first_element: '^[ ]\b(first_element)\b'
 last_element: '^[ ]\b(last_element)\b'
-check_if: '^[ ](\bcheck_if\b)(.*\belse do_check\b)'
+check_if: '^[ ](\bcheck_if\b)(.*\belse do_check\b)'
+is_set: '^[ ]\b(is_set)\b'