\begin{definition}[Definition:Formal Grammar/Top-Down]
Let $\LL$ be a formal language whose alphabet is $\AA$.
A '''top-down grammar''' for $\LL$ is a formal grammar which allows well-formed formulas to be built from a single metasymbol.
Such a grammar can be made explicit by declaring that:
* A metasymbol may be replaced by a letter of $\AA$.
* A metasymbol may be replaced by certain collations labeled with metasymbols and signs of $\AA$.
From the words thus generated, those not containing any metasymbols are the well-formed formulas.
\end{definition}