Rule
From Gallium
Rules take parts of Camlp4 Grammar Extensions, expressed with the EXTEND syntax.
Merely a level is a list of rules. A rule is a list of symbols and a semantic action.
Example:
EXTEND Gram expression: [ "top" [ "let"; x = var; "="; e1 = expresssion; "in"; e2 = expression -> Let(_loc, x, e1, e2) ] ] ; END;;