What are Zhang's molecules?
Restating the definition to make the quantifiers easier to understand:


  A molecule is a finite stable approximable mapping, such that there exists a largest pair $(a,p) \in m$, such that for all other pairs $(b,q) \in m$ we have $b \subseteq a$ and $\{p\} \vdash q$.


In other words, a molecule is a mapping that closes off neatly, having a largest pair.