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defined(var [, /target])
defined(var) returns 0 if var has no
value (i.e., is of data class undefined) or, when
var is a pointer (Pointers), if the ultimate target
of var is undefined (class #class.undefined), and
1 otherwise.
Keyword /target has influence only if x is a pointer
or a parameter to a user-defined subroutine or function. If
x is a pointer, then defined(x, /target)
yields 1 if the pointer has a target (even if that target is ultimately
linked to an undefined value), and 0 otherwise.
If x is a parameter to a user-defined subroutine or function, then
defined(x, /target) yields 1 if the user specified an
argument for this parameter, even if this argument is itself an
undefined variable, and 0 otherwise. This property is inherited if the
parameter is used as an argument to another user-defined subroutine or
function (Arguments).