Something like (to be inserted before "We can assign a value to a variable by using a “variable=value” pair." in section Defaults of Subroutines):
You can use
Default to assign an
empty default value to a parameter, which can be tested with an
IsEmpty (of
IsNotEmpty) condition:
DefCall MySub P
Default P
If IsEmpty P
// P was empty, presumably left out
EndIf
EndDefCall
Note that the following two calls are indistiguishable from the perspective of the subroutine body:
Set EmptyVar
Call MySub
Call MySub $EmptyVar