procedure main()
nulltest("a",a)                # unassigned variables are null by default
nulltest("b",b := &null)       # explicit assignment is possible
nulltest("c",c := "anything")
nulltest("c",c := &null)       # varibables can't be undefined
end

procedure nulltest(name,var)
return write(name, if /var then " is" else " is not"," null.")
end