- 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

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`.