Write the function even_double_tree which takes in a treet and returns a new tree with only the even labels doubled.
Assume that the below tree functions are available to use:

tree(label, branches=[]) -> constructs a new tree
label(t) -> returns the label of given tree
is_tree(t) -> returns true if given argument is a tree
is_leaf(t) -> returns true if given tree has no branches
branches(t) -> returns branches of given tree

def even_double_tree(t):
"""
>>>t = tree(4. [tree(3), tree(6)])
>>> even_double_tree(t)
tree(8,[tree(3), tree(12)])
""""
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if ___________________:
return ______________
else:
return