Here is the verb i made to challenge another player to fight, but its supposed to ask the challengee if he wants to accept the challenge no the challenger. i know why its not working, but i dont know how to fix it. Can someone please help?

mob/verb/Challenge(mob/M in oview(1))
if(usr.challenge==0)
if(M.challenge==0)
switch(input("You have been challenged, will You accept?","challenge")in list("Yes","No"))
if("Yes")
usr<<"Your challenge has been accepted, prepare for battle."
M<<"You have accepted a challange, prepare for battle."
usr.verbs += /mob/A/verb/Attack
M.verbs += /mob/A/verb/Attack
usr.challenge = 1
M.challenge = 1
Lock(usr)
Lock(M)
M.turn = 1
if("No")
usr<<"Your challenge has been declined by [M]."
M<<"You have declined [usr]'s challenge."
else
usr<<"[M] is already in a battle"
else
usr<<"You are already in a battle."
mob/A/verb/Attack(mob/M in oview(1))
if(usr.challenge == 1)
if(M.challenge == 1)
if(usr.turn == 1)
var/damage = usr.MPL/10
M.HP-=damage
usr<<"You inflict [damage] damage on [M]."
M<<"[usr] inflicts [damage] on You."
usr<<"It is now [M]'s turn."
M<<"It is now your turn."
M.turn = 1
usr.turn = 0
else
usr<<"Its not your turn."
else
usr<<"[M] hasnt been challenged."
else
usr<<"[M] hasnt been challenged."
if(M.HP<=0)
usr.HP = usr.MHP
M.HP = M.MHP
M.Move(1,1,1)
M<<"You have been killed by [usr]."
usr<<"You have killed [M]."
Unlock(usr)
Unlock(M)
usr.challenge = 0
M.challenge = 0
usr.turn = 0
M.turn = 0
usr.verbs-=/mob/A/verb/Attack
M.verbs-=/mob/A/verb/Attack
usr.PL+=50
usr.MPL+=50
usr.HP+=20
usr.MHP+=20