Checking parent's class

How can I check what a parent MovieClip's class is?
I have a Terrain class that I only want to use a function when the parent movieclip is a specific class, so that the terrain object can be used by multiple classes, but only launch that specific function when the parent's class is "WorldBase". How can i detect this through the Terrain class?

PHP Code:

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function checkClass(theObject:Object) {
if (theObject is WorldBase) {
do...
}
else {
do...
} 


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or perhaps one of these will work without function:

PHP Code:

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if (theTerrain.parent is WorldBase) 


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PHP Code:

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if (theTerrain(parent) is WorldBase) 


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