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	to say whether or not the interval tree has been visited (and skip
	it when revisited), and the mark bit of the plist field to say
	whether or not that interval has been visited (and abort if
	revisited); don't try to use the plist mark bit for both
	meanings.
	* alloc.c (mark_interval_tree): Don't test if the interval tree
	has already been visited here; let the MARK_INTERVAL_TREE macro do
	that; avoid function call overhead.  Mark the interval tree as
	having been visited by setting TREE->parent's mark bit.
	(MARK_INTERVAL_TREE): If the tree has been visited (according to
	I->parent's mark bit), don't call mark_interval_tree.
	(gc_sweep): Rebalance the interval trees of those large strings
	which are still alive.  This also clears the mark bits of those
	trees' root intervals' parent fields.
	(compact_strings): Rebalance the interval tree of each small
	strings which is still alive.  This also clears the mark bits of
	that tree's root interval's parent field.  Since the string has
	moved, update the root interval's parent pointer to contain the
	new address.
	* lisp.h (struct interval): Doc fix; explain the roles of the mark
	bits of the parent and plist members.