How to understand "?" in probe alias?

[Nan Xiao]

Hi all,

Below is "Testing target variable available Example"
(https://sourceware.org/systemtap/SystemTap_Beginners_Guide/targetavailable.html):

probe vm.pagefault = kernel.function("__handle_mm_fault@mm/memory.c") ?,
                     kernel.function("handle_mm_fault@mm/memory.c") ?
{
        name = "pagefault"
        write_access = (@defined($flags) ? $flags & FAULT_FLAG_WRITE :
$write_access)
        address =  $address
}

How to understand "?" in probe alias? After checking Language
reference(https://sourceware.org/systemtap/langref/Components_SystemTap_script.html#SECTION00041000000000000000),
I can't find the explanations of it.

Thanks in advance!

Best Regards
Nan Xiao