This is documentation for Mathematica 7, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.1)

Notation

Notation[boxes DoubleLongLeftRightArrow expr]
parses any input box structure boxes internally as expr, and formats any expression matching expr as boxes in output.
Notation[boxesDoubleLongRightArrowexpr]
restricts Notation to only parsing.
Notation[boxesDoubleLongLeftArrowexpr]
restricts Notation to only formatting.