- Expression & Expression, see Expression -- the class of all expressions
- Expression & Holder, see Expression -- the class of all expressions
- Expression & Thing, see Expression -- the class of all expressions
- Holder & Expression, see Expression -- the class of all expressions
- Holder & Holder, see Expression -- the class of all expressions
- Thing & Expression, see Expression -- the class of all expressions
- ZZ & ZZ -- logical and

This operator may be used as a binary operator in an expression like `x&y`. The user may install binary methods for handling such expressions with code such as

X & Y := (x,y) -> ...

where `X` is the class of `x` and `Y` is the class of `y`.