src/Form/Container.php
public function getActiveFormPrevious()
    {
        $key = null;
        $actualKey = $this->getActiveFormActual();
        if (isset($actualKey)) {
            $forms = array_keys($this->forms);
            $formsFlip =  array_flip($forms);
            $index = $formsFlip[$actualKey];
            if (0 < $index) {
                $key = $forms[$index-1];
            }
        }
        return $key;
    }
public function getActiveFormPrevious()
    {
        $key = null;
        $actualKey = $this->getActiveFormActual();
        if (isset($actualKey)) {
            $forms = array_keys($this->forms);
            $formsFlip =  array_flip($forms);
            $index = $formsFlip[$actualKey];
            if (0 < $index) {
                $key = $forms[$index-1];
            }
        }
        return $key;
    }
get the form before the actual active
@return null