A binary operator expression.

Binary operations can be left-associative (a X b X c means (a X b) X c) or right-associative (a X b X c means a X (b X c)). To represent this in the type system, the left and right children of concrete classes usually have different types: One has the type of the current precedence level (see the PrecedenceXOperation aliases), one has the type of the precedence level below.

(Some binary operations have no precedence because they can’t be nested (for example, a == b == c is never allowed); in this case, both children have the type of the precedence level below.)