= Solution
A collection of <logical connectives> is adequate, or has <functional completeness>, if every <Boolean function> of finitely many <propositional variables> can be expressed by a formula using those connectives and variables, with the usual convention for nullary truth constants stated if those are included.
An adequate example is $\boxed{\{\neg,\to\}}$. An inadequate example is $\boxed{\{\to\}}$: formulas using <logical implication> alone preserve truth when every input is true, so cannot express <negation>. These examples concern truth functions, rather than mere syntactic formation of formulas.
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