Definition:Formal Grammar/Top-Down

Definition
Let $\mathcal L$ be a formal language whose alphabet is $\mathcal A$.

A top-down grammar for $\mathcal L$ is one which allows well-formed words to be built from a single metasymbol.

Such a grammar can be made explicit by declaring that:


 * A metasymbol may be replaced by a primitive symbol of $\mathcal A$.


 * A metasymbol may be replaced by an element of a specified collection of concatenations of metasymbols and signs of $\mathcal A$.

From the words thus generated, those not containing any metasymbols are the well-formed words.