What chain complex is denoted by a single $R$?
Solution 1:
To me it looks like the complex which has just an $R$ term in degree $0$. Hence with homological grading the degree $0$ SES looks like $$0\to R\to R\to 0\to 0$$ and degree $-1$ looks like $$0\to 0\to R\to R\to 0$$ which are both exact. At any other degree you just get a SES of zeroes.