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|context> => |context: simple binary tree>


left |x> => |a>
right |x> => |b>
text |x> => |start node>


left |a> => |c>
right |a> => |d>
text |a> => |first child node>


left |b> => |e>
right |b> => |f>
text |b> => |second child node>


left |c> => |g>
right |c> => |h>


left |d> => |i>
right |d> => |j>


left |e> => |k>
right |e> => |l>


left |f> => |m>
right |f> => |n>


child |*> #=> left |_self> + right |_self>
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