( ( If ( you ( ( rotate ( the ( arrow tail ) ) ) ( to head ) ) ) ) ( , ( ( ( ( ( ( by ( 180 degrees ) ) ( , ( ( ( the direction ) ( of ( the arrow ) ) ) ( is reversed ) ) ) ) , ) so ) ( ( a ( 180 ( degree rotation ) ) ) ( reverses ( the arrow ) ) ) ) . ) ) )
(ROOT (S (SBAR (IN If) (S (NP (PRP you)) (VP (VB rotate) (NP (DT the) (NN arrow) (NN tail)) (PP (TO to) (NP (NN head)))))) (, ,) (S (PP (IN by) (NP (CD 180) (NNS degrees))) (, ,) (NP (NP (DT the) (NN direction)) (PP (IN of) (NP (DT the) (NN arrow)))) (VP (VBZ is) (ADJP (JJ reversed)))) (, ,) (IN so) (S (NP (DT a) (CD 180) (NN degree) (NN rotation)) (VP (VBZ reverses) (NP (DT the) (NN arrow)))) (. .)))
If you rotate the arrow tail to head, by 180 degrees, the direction of the arrow is reversed, so a 180 degree rotation reverses the arrow.
(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.