Function RTBIS(X1, X2, XACC)
    JMAX = 40
    FMID = FUNC(X2)
    F = FUNC(X1)
    If F * FMID >= 0# Then
        Print "Root must be bracketed for bisection."
    End If
    If F < 0# Then
        RTBIS = X1
        DX = X2 - X1
    Else
        RTBIS = X2
        DX = X1 - X2
    End If
    For J = 1 To JMAX
        DX = DX * 0.5
        XMID = RTBIS + DX
        FMID = FUNC(XMID)
        If FMID <= 0 Then RTBIS = XMID
        If Abs(DX) < XACC Or FMID = 0 Then Exit Function
    Next J
    Print "too many bisections"
End Function