  We consider classes $\mathscr{G}^s ([0,1])$ of subsets of $[0,1]$, originally
introduced by Falconer, that are closed under countable intersections, and such
that every set in the class has Hausdorff dimension at least $s$. We provide a
Frostman type lemma to determine if a limsup-set is in such a class. Suppose $E
= \limsup E_n \subset [0,1]$, and that $\mu_n$ are probability measures with
support in $E_n$. If there is a constant $C$ such that \[\iint|x-y|^{-s}\,
\mathrm{d}\mu_n(x)\mathrm{d}\mu_n(y)<C\] for all $n$, then under suitable
conditions on the limit measure of the sequence $(\mu_n)$, we prove that the
set $E$ is in the class $\mathscr{G}^s ([0,1])$. As an application we prove
that for $\alpha > 1$ and almost all $\lambda \in (\frac{1}{2},1)$ the set \[
E_\lambda(\alpha) = \{\,x\in[0,1] : |x - s_n| < 2^{-\alpha n} \text{infinitely
often}\ \}\] where $s_n \in \{\,(1-\lambda)\sum_{k=0}^na_k\lambda^k$ and
$a_k\in\{0,1\}\,\}$, belongs to the class $\mathscr{G}^s$ for $s \leq
\frac{1}{\alpha}$. This improves one of our previous results.
