| ?- aprove(lit(L)).
Is up(s2) true? [yes, no, unknown]
|: yes.
Is up(s1) true? [yes, no, unknown]
|: no.
Is down(s2) true? [yes, no, unknown]
|: no.
Is up(s3) true? [yes, no, unknown]
|: yes.
L = l2 ? 
yes
