Error "You cannot make a category of one of its own subcategories."

Hi! When copying one category to another, the error "You cannot make a category of one of its own subcategories." On closer examination, it turned out that if the category ID in which I copy, for example "320", then categories with ID 3 and 32 cannot be copied into it. This is checked by the condition in the file course / management.php in line 316:

if (strpos ($movetocat > path, $cattomove> path) === 0) {

It is not very clear why there is a condition of the form that checks the entry of the path?