code: def normals(triangles=None, crosses=None):
      
      if crosses is None:
          crosses = cross(triangles)
      # unitize the cross product vectors
      unit, valid = util.unitize(crosses, check_valid=True)
      return unit, valid
docs: Calculates the normals of input triangles
  
      Parameters
      ------------
      triangles : (n, 3, 3) float
        Vertex positions
      crosses : (n, 3) float
        Cross products of edge vectors
  
      Returns
      ------------
      normals : (m, 3) float
        Normal vectors
      valid : (n,) bool
        Was the face nonzero area or not