Function RTNEWT(X1, X2, XACC)
    JMAX = 20
    RTNEWT = 0.5 * (X1 + X2)
    For J = 1 To JMAX
        Call FUNCD(RTNEWT, F, DF)
        DX = F / DF
        RTNEWT = RTNEWT - DX
        If (X1 - RTNEWT) * (RTNEWT - X2) < 0 Then
            Print "jumped out of brackets"
            Exit Function
        End If
        If Abs(DX) < XACC Then Exit Function
    Next J
    Print "RTNEWT exceeding maximum iterations."
End Function