Performs pre-order traversal without recursive function calls.
When a leaf node is first encountered, func is called with the leaf node as its argument, and its result is appended to the list.
For example, the statement:
ids = root.pre_order(lambda x: x.id)
returns a list of the node ids corresponding to the leaf nodes of the tree as they appear from left to right.
Parameters : | func : function
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Returns : | L : list
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