quadtree.x([x])
If x is specified, sets the current x-coordinate accessor and returns the quadtree. If x is not specified, returns the current x-accessor, which defaults to:
function x(d) { return d[0]; }
The x-acccessor is used to derive the x-coordinate of data when adding to and removing from the tree. It is also used when finding to re-access the coordinates of data previously added to the tree; therefore, the x- and y-accessors must be consistent, returning the same value given the same input.
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