( ( a ( double-headed arrow ) ) ( ( indicates ( that ( ( ( a sequence ) ( of values ) ) ( is expected ) ) ) ) . ) )
(ROOT (S (NP (DT a) (JJ double-headed) (NN arrow)) (VP (VBZ indicates) (SBAR (IN that) (S (NP (NP (DT a) (NN sequence)) (PP (IN of) (NP (NNS values)))) (VP (VBZ is) (VP (VBN expected)))))) (. .)))
a double-headed arrow indicates that a sequence of values is expected.
(ROOT (S (NP (JJ Reversible) (NNS reactions)) (VP (VBP are) (VP (VBN indicated) (PP (IN by) (NP (NP (JJ opposite-headed) (NNS kinds)) (PP (IN of) (NP (NNS arrows))))))) (. .)))
Reversible reactions are indicated by opposite-headed kinds of arrows.