= Solution
The <unique readability theorem> says that every <propositional formula> has \b[exactly one] of the following forms: a unique <propositional variable>; $\neg\psi$ for a unique <logical formula> $\psi$; or $(\psi\to\chi)$ for a unique <ordered pair> of <logical formulas> $(\psi,\chi)$. These three alternatives are disjoint. Consequently each <logical formula> has a unique <formula syntax tree> and a uniquely determined principal <logical connective> whenever it is not a <logical variable>.
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