That's what I thought too!:
```
>>> from iris._constraints import *
>>> _ColumnIndexManager
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '_ColumnIndexManager' is not defined
>>> from iris._constraints import _ColumnIndexManager
>>> _ColumnIndexManager
<class 'iris._constraints._ColumnIndexManager'>
```