Such requirements could arise either as a result of voluntary agreements among retailers and apparel suppliers or be mandated through regulation.
( ( Such requirements ) ( ( could ( ( ( ( arise either ) ( as ( ( a result ) ( of ( ( voluntary agreements ) ( among ( retailers ( and ( apparel suppliers ) ) ) ) ) ) ) ) ) or ) ( be ( mandated ( through regulation ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Such) (NNS requirements)) (VP (MD could) (VP (VP (VB arise) (ADVP (RB either)) (PP (IN as) (NP (NP (DT a) (NN result)) (PP (IN of) (NP (NP (JJ voluntary) (NNS agreements)) (PP (IN among) (NP (NNS retailers) (CC and) (NN apparel) (NNS suppliers)))))))) (CC or) (VP (VB be) (VP (VBN mandated) (PP (IN through) (NP (NN regulation))))))) (. .)))
One of these requirements is no agreement not to undercut each others prices. 
( ( One ( of ( these requirements ) ) ) ( ( is ( no ( agreement ( not ( to ( undercut ( each ( others prices ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (CD One)) (PP (IN of) (NP (DT these) (NNS requirements)))) (VP (VBZ is) (NP (DT no) (NN agreement) (S (RB not) (VP (TO to) (VP (VB undercut) (NP (DT each) (NNS others) (NNS prices))))))) (. .)))