The Bison declarations section of a Bison grammar defines the symbols used in formulating the grammar and the data types of semantic values. See Symbols.
All token type names (but not single-character literal tokens such as
'*') must be declared. Nonterminal symbols must be
declared if you need to specify which data type to use for the semantic
value (see More Than One Value Type).
The first rule in the grammar file also specifies the start symbol, by default. If you want some other symbol to be the start symbol, you must declare it explicitly (see Languages and Context-Free Grammars).