- 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`.

This operator may be used as a prefix unary operator in an expression like `<==y`. The user may install a method for handling such expressions with code such as

<== Y := (y) -> ...

where `Y` is the class of `y`.