Peter Linz - An introduction to formal languages and automata (2001) third edition:
A context free grammar $G=(V,T,S,P)$ is said to be a simple grammar or s-grammar if all its productions are of the form
$A \rightarrow aX$
where $A \in V, a \in T, x \in V^*$ , and any pair $(A,a)$ occurs at most once in $P$.
Define a simple grammar or s-grammar for $L_1=\{a^nb^n, n \ge 1\}$ is not difficult.
$S \rightarrow aY$
$Y \rightarrow aYB | b$
$B \rightarrow b$
How about this Language: $L_2=\{a^nb^m, n > m \ge 0\}$ . Is there a s-grammar for this language?