This function is the primary function called upon by
<< to format expressions for printing. The default method provided by the system is to convert
x to an
Expression with
expression and then to convert that to a net.
A new method for formatting expressions of class
X may be installed by the user with code of the form
net X := x -> .... The function provided by the user should return a net or a string.
There are various such methods for formatting, but we don't document them separately.