MERTON1987的a simple model of capital market equilibrium with incomplete information 中有一小段不了解就是第487页(JOF,1987)的第三个公式那里

In addition to shares in the firms, there are two other traded securities:a riskless security with sure return per doller R and a security that combines the riskless security and a forward comtract with cash settlement on the observed factor index Y.without loss of generality, it is assumed that the forward price of the contract is such that the standard of the equilibrium return on the security is unity. Thus, the return on the security can be written as:

(3) R(n+1)=E(R(n+1))+y

加入的两个证券的区别在哪里呢?

另外without loss of generality, it is assumed that the forward price of the contract is such that the standard of the equilibrium return on the security is unity. 尤其是这句话,没弄懂