Make XOX private

It's intended only for use within `is_versor`

Every member we can remove from `Ga` makes galgebra easier to understand

Author: eric-wieser

galgebra/ga.py

@@ -447,7 +447,7 @@ def __init__(self, bases, **kwargs):
         self.a = []  # List of dummy vectors for Mlt calculations
         self._agrads = {}  # cache of gradient operator with respect to vector a
         self.dslot = -1  # args slot for dervative, -1 for coordinates
-        self.XOX = self.mv('XOX','vector')  # Versor test vector
+        self._XOX = self.mv('XOX','vector')  # cached vector for use in is_versor
 
     def make_grad(self, a, cmpflg=False):  # make gradient operator with respect to vector a
 

galgebra/mv.py

@@ -981,7 +981,7 @@ def is_versor(self):
         if not test.is_scalar():
             return self.versor_flg
         # see if self*x*self.rev() returns a vector for x an arbitrary vector
-        test = self * self.Ga.XOX * self.rev()
+        test = self * self.Ga._XOX * self.rev()
         self.versor_flg = test.is_vector()
         return self.versor_flg