To make sure a public variable can be created regardless if it has been used before, use this:

** ChkPrivateVariable.prg
Private zuretval
zuretval="PrivateVariable"
Suspend
**EOF

** Now in command window :
** 1. run this prg file (do ChkPrivateVariable)
** 2. Try to declare the same variable as Local OR Public 

Release zuretval
Public zuretval

** Here it will no longer show the same error.