You can declare an instance of a class inside itself (very useful for most data structures).
One of the ways you can get around problems this might cause in teh .h being included multiple times is to put in a failsafe....
in Spline.h, surround exerything in the file with:
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#ifndef _SPLINE_H
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#define _SPLINE_H
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/* Do all the normal things here */
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#endif
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This means when Spline.h is included for the first time you set a value to show that you have already declared this header, and it won't be declared again.