Determine if a given name is a PUBLIC or PRIVATE memory variable
LOCAL TheLocal
STATIC TheStatic
PUBLIC ThePublic
PRIVATE ThePrivate
? __MVEXIST( "NotExist" ) // .F.
? __MVEXIST( "TheLocal" ) // .F.
? __MVEXIST( "TheStatic" ) // .F.
? __MVEXIST( "ThePublic" ) // .T.
? __MVEXIST( "ThePrivate" ) // .T.