Every expression with the ! operator becomes a call to
this function, i.e. !a is the same as
predef::`!(a).
It's also used when necessary to test truth on objects, i.e. in
a statement if (o) ... where o is an object, the
test becomes the equivalent of !!o so that any
lfun::`!() the object might have gets called.
Returns
If arg is an object that implements lfun::`!() , that function
will be called.
If arg is 0 (zero), a destructed object, or a function in a
destructed object, 1 will be returned.