== empty ==
fun empty() {}
---------------------
L0:
    <START>        NEXT:[read (Unit)]    PREV:[]
    read (Unit)    NEXT:[<END>]          PREV:[<START>]
L1:
    <END>          NEXT:[<SINK>]         PREV:[read (Unit)]
error:
    <ERROR>        NEXT:[<SINK>]         PREV:[]
sink:
    <SINK>         NEXT:[]               PREV:[<ERROR>, <END>]
=====================