We consider the rate sum of a symmetric Gaussian interference channel in
standard format, 
given by $Y_1=X_1+aX_2+Z_1$ and $Y_2=aX_1+X_2+Z_2$,
with $X_1$ and $X_2$ constrained to average power $P$, and $Z_1$ and $Z_2$
distributed as $N(0,1)$. 
We provide a complete taxonomy of potentially optimal Gaussian signaling
strategies in the entire weak and moderate interference parameter space,
delimited by $0<a<1$ and $P>0$.
The strategies seem to lead to the Han and Kobayashi achievable region for the
rate sum. We find that the parameter space comprises
eight (potentially) optimal transmission regions formed by four pure mode
strategies and four transitional strategies. 
We characterize the boundaries of these pure mode regions and the associated
transitional regions.