How to check for optional argument?

[kelvinkoko]

Sorry it maybe a simple question, but i seems cannot find the answer in doc
For example i have this grammar
If = "if" StatementInRound StatementInCurly Else?
The Else is optional,
so in semantics, what is the recommend way to check whether this match contain "Else " case?

// Semantic section: 
If(_if, condition, yesStatements, elseStatements) {
// How i can check for whether elseStatements is  included or not?  
}

[gmhta]

elseStatements.numChildren === 0