src/Checkout/CheckoutFieldFactory.php
private function getSubset(FieldList $fields, $subset = array())
    {
        if (empty($subset)) {
            return $fields;
        }
        $subfieldlist = FieldList::create();
        foreach ($subset as $field) {
            if ($field = $fields->fieldByName($field)) {
                $subfieldlist->push($field);
            }
        }
        return $subfieldlist;
    }
private function getSubset(FieldList $fields, $subset = array())
    {
        if (empty($subset)) {
            return $fields;
        }
        $subfieldlist = FieldList::create();
        foreach ($subset as $field) {
            if ($field = $fields->fieldByName($field)) {
                $subfieldlist->push($field);
            }
        }
        return $subfieldlist;
    }
Helper function for reducing a field set to a given subset,
in the given order.

@param FieldList $fields form fields to take a subset from.
@param array     $subset list of field names to return as subset

@return FieldList subset of form fields