+ /**
+ * Get length of shortest whitespace prefix (at the start of a line).
+ *
+ * If there is a line with no prefix whitespace, 0 is a valid return value.
+ *
+ * @param string $str String to check
+ * @return int Length in characters. Tabs count as single characters.
+ */
+ private function getShortestWhitespacePrefixLen(string $str) : int {