The empty type is defined in the directory lib_empty.
** Module lib_empty Imports parameters
$empty : Type(empty)
$empty_elim : {C_empty:empty->TYPE}{z:empty}C_empty z
** Label (!empty!) empty
** Label (!empty elim!) empty_elim
empty_rec = ... : {t|Type}empty->t
empty_ind = ... : {P:empty->Prop}{z:empty}P z