OptionalElement
OptionalElement[patt]
is a grammar rules pattern object that represents 0 or 1 instances of patt.
OptionalElement[patt,default]
uses default if the element is not present.
Examples
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Wolfram Research (2015), OptionalElement, Wolfram Language function, https://reference.wolfram.com/language/ref/OptionalElement.html.
Text
Wolfram Research (2015), OptionalElement, Wolfram Language function, https://reference.wolfram.com/language/ref/OptionalElement.html.
CMS
Wolfram Language. 2015. "OptionalElement." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/OptionalElement.html.
APA
Wolfram Language. (2015). OptionalElement. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/OptionalElement.html