Black, Fischer and John C. Cox, "Valuing Corporate Securities: Some Effects of Bond Indenture Provisions", Journal of Finance, Vol. 31, No. 2, (May 1976), pp. 351-367.
Abstract: In a recent paper Black and Scholes  presented an explicit equilibrium model for valuing options. In this paper they indicated that a similar analysis could potentially be applied to all corporate securities. In other papers, both Merton  and Ross [REF] noted the broad applicability of option pricing arguments. At the same time Black and Scholes also pointed out that actual security indentures have a variety of conditions that would bring new features and complications into the valuation process.