is there a way to revert os.pullEvent = os.pullEventRaw?

--Drachma 0.1

os.pullEvent=os.pullEventRaw

--[[to-do list:
logout feature
create "user already exists" portion of newUser
develop os functions
create password changing function
admin password cannot be changed
experiment w/ h.write & h.writeLine commands
create file identification
create program licenses
admins grant licenses to .iso files to authorize their use
licenses must be verified before every attempt to run program
dangerScan level 1 failures ignored
develop dangerScan license compatibility

big: develop Drachma as OS
big: develop Wenceslas
]]

--[[file system extensions
.prg = program
.dba = database
.txt = text
.iso = isolated program
.lic = licensed program
.img = image
]]

--Variable lineup
userTrust = false
adminTrust = false
aiCheck = false

--Functions
function login()
term.clear()
term.setCursorPos(1,1)
print("Username: ")
term.setCursorPos(1,3)
usr = read()
term.setCursorPos(1,3)
print("Password: ")
term.setCursorPos(1,4)
pwd = read("*")
usr.dba = fs.open("drachma/usr.dba", "r")
pwd.dba = fs.open("drachma/pwd.dba", "r")
_usr=textutils.unserialize(usr.dba readAll())
_pwd=textutils.unserialize(pwd.dba.readAll())
for i = 1,0,1 do
  if usr = _usr[i] and pwd = _pwd[i] then
   if usr="admin" then
	adminTrust = true
	os.pullEvent = os.pullEvent
	usertrust = true
	i = 0
	home()
   else
	userTrust = true
	i = 0
	home()
   end
  else
  print("Credential Mismatch. Access Denied.")
  end
end
end

function gui()
end

function aiStatus()
term.clear()
if aiCheck == true then
  shell.run("drachma/Wenceslas.lai")
else
  x, y = term.getSize()
  _x = x-14
  term.setCursorPos(_x,y)
  io.write("AI unavailable.")
  term.setCursorPos(1,1)
end
end

function os()
input = read()
if string.sub(input,1,4)=="open"
  --extensions(input,5,#input)
else
  print("Unrecognized Command.")
  sleep(3)
end
end

function home()
while true do
  --gui()
  aiStatus()
  --os()
end
end

function newUser()
if adminTrust==true then
  term.setCursorPos(1,1)
  print("New Username:")
  term.setCursorPos(1,2)
  usr = read()
  term.setCursorPos(1,3)
  print("New Password:")
  term.setCursorPos(1,4)
  pwd = read("*")
  usr.dba = fs.open("drachma/usr.dba", "a")
  usr.dba.writeLine(usr)
  usr.dba.flush()
  usr.dba.close()
  pwd.dba = fs.open("drachma/pwd.dba", "a")
  pwd.dba.writeLine(pwd)
  pwd.dba.flush()
  pwd.dba.close()
  print("Success! User".. usr .." created.")
  sleep(5)
  term.clear()
else
  print("Access Denied. Administrator priveleges required.")
  sleep(5)
  term.clear()
end
home()
end

function dangerScan(fileName)
file = fs.open(fileName,"r")
contents = file.readAll()
lv2.dba = fs.open("drachma/lv2.dba", "r")
lv2 = textutils.unserialize(lv2.dba.readAll())
lv1.dba = fs.open("drachma/lv1.dba", "r")
lv1 = textutils.unserialize(lv1.dba.readAll())
for i = 1, #lv2 do
  status = string.search(contents, lv2[i])
  if status ~= nil then
   fs.delete(fileName)
   print("Dangerous script detected, deleting file.")
   return(2)
  else
   status = string.search(contents, lv1[i])
   if status ~= nil then
	fs.move(fileName, string.sub(fileName, 1, #fileName-3).."iso")
	print("Suspicious script detected, isolating file.")
	return(1)
   else
	return(0)
   end
  end
end
end

login()

local oldPull = os.pullEvent
os.pullEvent = os.pullEventRaw
...
os.pullEvent = oldPull

if not string.find( user.dba.readAll(), usr) then

function override(host, functionName, f, prevRevert)
    local old = host[functionName]
    host[functionName] = f
    return function()
        if prevRevert then
            prevRevert()
        end
        host[functionName] = old
    end
end


local revert = override(os, "pullEvent", os.pullEventRaw)
revert = override(something, "else", otherElse, revert)
...
revert()