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How about the top half of a circle, counter-clockwise, with the tip of an
arrow;
basically, this is also the icon offered for the "UNDO" function, e.g. under
Winword,
which is pretty much also a kind of backtracking: revert one step back. If
you want to stress the backtracking path, a sequence of maybe three
half-circles with arrow tips might be suitable. Let us know in time what
you've finally decided on.