( ( ( ( The head ) ( of ( the arrow ) ) ) ( indicates ( ( the ( final position ) ) ( of ( an ( electron pair ) ) ) ) ) ) ( , ( ( ( the tail ) ( of ( the arrow ) ) ) ( ( indicates ( which ( ( electron pair ) ( is ( being relocated ) ) ) ) ) . ) ) ) )
(ROOT (S (S (NP (NP (DT The) (NN head)) (PP (IN of) (NP (DT the) (NN arrow)))) (VP (VBZ indicates) (NP (NP (DT the) (JJ final) (NN position)) (PP (IN of) (NP (DT an) (NN electron) (NN pair)))))) (, ,) (NP (NP (DT the) (NN tail)) (PP (IN of) (NP (DT the) (NN arrow)))) (VP (VBZ indicates) (SBAR (WHNP (WDT which)) (S (NP (NN electron) (NN pair)) (VP (VBZ is) (VP (VBG being) (VP (VBN relocated))))))) (. .)))
The head of the arrow indicates the final position of an electron pair, the tail of the arrow indicates which electron pair is being relocated.
(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.