Head part of file, before any named section can contain common definitions. This section can also be declared explicitly using %%defs keyword. Common definitions section contains general declarations and options common for lexer and parser.
Lexer-specific and parser-specific definitions should be placed in %%ldefs and %%pdefs sections respectively.
For backward compatibility reasons, it allowed to put lexer and parser definitions also in common %%defs section. This practice should be avoided in newly created files.