NAME
	SetFunction -- change a function vector in a library

   SYNOPSIS
	oldFunc = SetFunction(library, funcOffset, funcEntry)
	D0		      A1       A0.W	   D0

	APTR SetFunction(struct Library *,LONG,APTR);

   FUNCTION
	SetFunction is a functional way of changing where vectors in a
	library point.	They are changed in such a way that the
	checksumming process will never falsely declare a library to be
	invalid.

   WARNING
	If you use SetFunction on a function that can be called from
	interrupts, you are obligated to provide your own arbitration.

   NOTE
	SetFunction cannot be used on non-standard libraries like pre-V36
	dos.library.  Here you must manually Forbid(), preserve all 6
	original bytes, set the new vector, SumLibrary(), then Permit().

   INPUTS
	library    - a pointer to the library to be changed
	funcOffset - the offset of the function to be replaced
	funcEntry  - pointer to new function

   RESULTS
	oldFunc    - pointer to the old function that was just replaced

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