xml DTD correctness, why would we use round braces inside of round braces when declaring XML node? -

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i'm looking @ piece of xml (inside of dtd file):

<!element entry ((node1?, node2?, node3?), node4, node5?, (node6 | node16)?, (node17, node18?, (node19, node20)?)*, node21?, node22?, node23*, node24*, ((node25) | (node26?, node27?))?, node28*, node29?, node30*, node31*)>

what reasoning behind having (node1?, node2?, node3?) in round braces? mean node1 , node2 , node3 optional? -> aren't alreday optional fact have question mark added already?

parentheses used grouping. grouping lets put qualifiers on parts of patterns, (node6 | node16)?, or control precedence, (node25) | (node26?, node27?). in case of (node1?, node2?, node3?), parentheses redundant. means same thing node1?, node2?, node3?.

sometimes people add parentheses because think makes things easier read, or emphasize related pieces of pattern. people type no reason.


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