<=== -- a unary and binary operator

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.