<==> -- a binary operator

Ways to use symbol <==> :

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

For the programmer

The object <==> is a keyword.

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.