'What does 'expr' and '\+' in Yocto class do

Within a Yocto class definition I find the following:

$(expr ${BOOT_SPACE_ALIGNED} \+ ${IMAGE_ROOTFS_ALIGNMENT})

I assume 'expr' is the bash command, but what does the '\+' do ? With multiplication the '*' must be escaped, but this does not apply to '+' as far as I know.



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