question

If you want to know whether the label exists and has a "truthy" value, you can use the question mark:

if(current.maynotexist?) {
 // current.maynotexist exists and is non-zero, or points at a valid object
}

If you just want to know if it exists, you can do

treenode maynotexist = current.labels["maynotexist"];
if( maynotexist ) {
  // the node exists, get its value with maynotexist.value
}

However, you don't need the latter construction very often, for example, if you want to create the label if it does not exist, you can

current.labels.assert( "maynotexist" )

and if you assign a label with

current.maynotexist = 1;

it will be automatically created.