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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