Consider a nondeterministic PDA $P$ which pushes/pops at most one stack symbol on a transition. Suppose that for every string $\sigma \in L(P)$, there is an accept computation of $\sigma$ in $P$ which starts with an empty stack and ends with an empty stack and which has the additional property that during this computation, the stack size can increase, but once the stack size decreases it cannot subsequently increase. I want to show that there is a CFG equivalent to $P$ with the property that for each production in this grammar, there is at most one variable on its right-hand side.
My initial idea to approach this problem was to convert $P$ into an equivalent PDA $P'$ which either pops or pushes on every transition, but never both, which empties the stack before accepting, and which has a single final state. Then I would convert $P'$ into a CFG $G'$ using the standard method, as seen here. Finally, I could show that for every $\sigma \in L(P') = L(P)$, there exists a derivation of $\sigma$ in $G$ such that every production applied in this derivation is of the form $A_{pq} \rightarrow aA_{rs}b$, where $p,q,r,s$ are states in $P'$ with the property that a transition exists from $p$ to $r$ which reads input symbol $a$ and pushes a symbol $u$ onto the stack and a transition exists from $s$ to $q$ which reads input symbol $b$ and pops $u$. However, I don't know how to show this to be the case.
