local function findPeripheral(_type, errMsg, callback)
  callback = type(callback) == "function" and callback or function() return true end
  local results = {}
  for _,name in pairs(peripheral.getNames()) do
    if peripheral.getType(name) == _type then
      local periph = peripheral.wrap(name)
      if callback(name, periph) then
        results[#results+1] = periph
      end
    end
  end
  if #results > 0 then
    return unpack(results)
  end
  error(errMsg or "Cannot find ".._type.." attached to this computer", 0)
end