let $a\in \mathbb{R}$ and let X and Y be independent normally distributed random variables, with mean $0$ and respective variances $\sigma^2_X$ and $\sigma^2_Y$. Can we express $$P(X+Y<a\,|\,X<a)$$ with a simple formula (say elementary functions + erf)?
This question is related to this more general one, which hasn't been answered.