The **RNA7D** model (Tillier and Collins, 1998) is a biologically plausible restriction of
the **RNA7A** model. The restictions in the **7D** model are analogous to the restrictions
made in the **6B**. There is one more frequency parameter for the mismatch state and one more rate
ratio parameter for the substitution rates involving this state.
The reference for the rate ratios are the rate of double transitions.
This model is described by the following rate matrix (table 2.9).

Gowri-Shankar Vivek 2003-04-24