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   Finds succesor nodes in a square grid.
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   Form:
   successorNodes = GridSuccessorNodes( node, parent, d )
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   Inputs
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   startNode     (1,1) Id of start location
   goalNode      (1,1) Id of next location
   d             (1,1) Data structure
                       .n      (1,1) Number of nodes

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   Outputs
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  successorNodes (1,:)  List of nodes in path 

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