system/Validation/Rules.php
public function in_list(string $value = null, string $list, array $data): bool
	{
		$list = explode(',', $list);
		$list = array_map(function ($value) {
			return trim($value);
		}, $list);
		return in_array($value, $list, true);
	}
public function in_list(string $value = null, string $list, array $data): bool
	{
		$list = explode(',', $list);
		$list = array_map(function ($value) {
			return trim($value);
		}, $list);
		return in_array($value, $list, true);
	}
Value should be within an array of values

@param  string $value
@param  string $list
@param  array  $data
@return boolean