Impossible.
( Impossible . )
(ROOT (FRAG (ADJP (JJ Impossible)) (. .)))
Impossible, unless circumstances are met.
( Impossible ( , ( ( unless ( circumstances ( are met ) ) ) . ) ) )
(ROOT (FRAG (ADJP (JJ Impossible)) (, ,) (SBAR (IN unless) (S (NP (NNS circumstances)) (VP (VBP are) (VP (VBN met))))) (. .)))