my controller fails to find helper function

Hi Team,
I love the new error interface - great job!
Its helping me to upgrade my ci3 to ci 4.6.0
Latest snag is my controller which extends BaseController cannot find the validation function:
- the function is 'is_valid_level' which is defined in a Helper myvalidate_helper.php
- the myvalidate helper is loaded (I think) by adding to the $helpers array in the BaseController (I can see it in the list of files).
- ci3 call was

PHP Code:

if (!is_valid_level($level3)) { .... 


- and I changed it to after a bit of research

PHP Code:

if (!myvalidate(is_valid_level($level3))) { .... 


- and the error reported is: Call to undefined function App\Controllers\myvalidate()
Clearly I have misunderstood something important, so could someone explain what I have got wrong please?
Thx Paul