 Solution

Use direct-transfer STRIPS actions: one action includes picking up a block and putting it down again. There is no separate holding state. Ground $b,y$ as distinct blocks and $x$ as a support, either a block or the table, with $x\ne b$ and $x\ne y$.

For $\mathrm{Move}(b,x,y)$, takeThe preconditions ensure that $b$ can be picked up and that the destination has room. Moving it frees a block source and occupies the destination, while $\mathrm{Clear}(b)$ persists. When $x=\tau$, table availability remains true; its redundant addition is simply omitted. Since $x$ is fixed in each ground action, this case distinction constructs ordinary add lists rather than introducing conditional action effects. The search transition model is $(S\setminus \mathrm{Del})\cup \mathrm{Add}$.