!3 You can manipulate the list of loaded cell handlers using the !-CellHandlerLoaderFixture-!.
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Here are the defaults.

!|cell handler inspector|
|Name|
|DefaultCellHandler|
|BlankKeywordHandler|
|NullKeywordHandler|
|ErrorKeywordHandler|
|EmptyCellHandler|
|BoolHandler|
|SymbolSaveHandler|
|SymbolRecallHandler|
|FailKeywordHandler|
|ExceptionKeywordHandler|

Now clear the handlers..
!|cell handler loader|
|clear|

and the list should be empty.
!|cell handler inspector|
|Name|

No reload the defaults..
!|cell handler loader|
|loadDefaults|

and they should all be in the list.
!|cell handler inspector|
|Name|
|DefaultCellHandler|
|BlankKeywordHandler|
|NullKeywordHandler|
|ErrorKeywordHandler|
|EmptyCellHandler|
|BoolHandler|
|SymbolSaveHandler|
|SymbolRecallHandler|
|FailKeywordHandler|
|ExceptionKeywordHandler|

Add an optional handler..
!|cell handler loader|
|load|SubstringHandler|

and it should be in the list.
!|cell handler inspector|
|Name|
|DefaultCellHandler|
|BlankKeywordHandler|
|NullKeywordHandler|
|ErrorKeywordHandler|
|EmptyCellHandler|
|BoolHandler|
|SymbolSaveHandler|
|SymbolRecallHandler|
|FailKeywordHandler|
|ExceptionKeywordHandler|
|SubstringHandler|

Remove a single handler..
!|cell handler loader|
|remove|SubstringHandler|

and it should not be in the list.
!|cell handler inspector|
|Name|
|DefaultCellHandler|
|BlankKeywordHandler|
|NullKeywordHandler|
|ErrorKeywordHandler|
|EmptyCellHandler|
|BoolHandler|
|SymbolSaveHandler|
|SymbolRecallHandler|
|FailKeywordHandler|
|ExceptionKeywordHandler|