StructureList:

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   Creates a list of structures and variables.

   d may contain data structures or any type of variable in its cell
   array. On initialization it returns the handle to the list. Also
   'modifier' is a call back string that StructureList should call when an
   element in the list is selected. When an element in a list is selected
   an array is created with pointers to the element and to all structures
   to which it is attached. This can be used to identify the element in a
   structure.
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   Form:
   s = StructureList( action, modifier, d, name, position, hFig )
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   ------
   Inputs
   ------
   action          (1,:)   Action to be taken 
   modifier        (1,:)   Modifier to the action
   d               {}      Cell array of data structures
   name            {}      Cell array of data structure name
   position        (1,4)   Position of the frame
   hFig            (1,1)   Handle to the figure

   -------
   Outputs
   -------
   hL              (1,:)   Handle to the selected field
   tag             (1,:)   StructureList tag

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