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You want to modify the output of some wp core function and instead of modifying the core directly (which is always bad), you have found a call like this:
$filterable_var = apply_filters( 'name_of_filter', $filterable_var );.
The filter function allows you to modify eg.
$filterable_var and therefore the output of the wp function.
You don't know what exactly the function wants as input.
Check what you can modify.
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