[TYPO3-english] RTE configuration in extension via TCA to only show a subset of buttons

[Roland]

hi everybody,

task: to configure the RTE via TCA for a specific column in one of my 
extensions that only the bold-button should be shown

[OK] if i use this snippet all buttons are available in the RTE:

--- quote ---
'defaultExtras' => 'richtext[*]:rte_transform[mode=ts_css]'
--- /quote ---

[OK] if i use this snippet the buttons defined by the default 
configuration are available in the RTE:

--- quote ---
'defaultExtras' => 'richtext[]:rte_transform[mode=ts_css]'
--- /quote ---

[PROBLEM] this snippet unexpectetly also shows the buttons defined by 
the defualt configuration:

--- quote ---
'defaultExtras' => 'richtext[bold]:rte_transform[mode=ts_css]'
--- /quote ---

what could be the problem here?

kind regards

roland