clone method
Implementation
@override
DocumentDirective clone() {
var clonedFunctions = <LiteralTerm>[];
for (var function in functions) {
clonedFunctions.add(function.clone());
}
var clonedGroupRuleBody = <TreeNode>[];
for (var rule in groupRuleBody) {
clonedGroupRuleBody.add(rule.clone());
}
return DocumentDirective(clonedFunctions, clonedGroupRuleBody, span);
}