How can I exchange two arbitrary subtrees of the same tree, i.e.
exchange-tree : TREE1, TREE2 -> WHATEVER
regardless of their relative position within the surrounding tree.
Annotations of both of the trees also have to be swapped around:
/\ /\
/ \ exchangeex-tree / \
/ \ ---------------> / \
/x____y\ /x____y\
| | | |
a1-T1 T2-a2 a2-T2 T1-a1
Thanks for any hint
Guido