function klass()
 local k={}
 k.__index=k
 function k:new(...)
  local n={}
  setmetatable(n,k)
  n:init(...)
  return n
 end
 return k
end

function assert_range(i,mn,mx,name)
 assert(i >= mn and i < mx, name.." must be ["..mn..","..mx..")")
end