A mutable set class that tracks the selection of multi-select
`~lsst.pex.config.ConfigChoiceField` objects.
Parameters
----------
dict_ : `ConfigInstanceDict`
The dictionary of instantiated configs.
value
The selected key.
at : `lsst.pex.config.callStack.StackFrame`, optional
The call stack when the selection was made.
label : `str`, optional
Label for history tracking.
setHistory : `bool`, optional
Add this even to the history, if `True`.
Notes
-----
This class allows a user of a multi-select
`~lsst.pex.config.ConfigChoiceField` to add or discard items from the set
of active configs. Each change to the selection is tracked in the field's
history.