Add the "or" keyword
Actually only the "and" keyword can be used. For example "Chain A and backbone" will select the backbone atoms only for the chain A. If I want to select the backbone for chain A and chain B but not for chain C, I can use "Chain A to B and backbone".
But, if I want the backbone of the chain A and the backbone of the chain C but not the chain B I can't ! It could be write as "(Chain A and backbone) or (Chain C and backbone)".
The implementation could use parenthesis to separate the different groups.