Stores processing information about a field in $form_state.
$parents: The array of #parents where the field lives in the form.
$field_name: The field name.
$langcode: The language in which the field values are entered.
$form_state: The form state.
$field_state: The array of data to store. See field_form_get_state() for the structure and content of the array.
function field_form_set_state($parents, $field_name, $langcode, &$form_state, $field_state) {
$form_state_parents = _field_form_state_parents($parents, $field_name, $langcode);
drupal_array_set_nested_value($form_state, $form_state_parents, $field_state);
}