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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def normals(triangles=None, crosses=None):
 """ 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 """

    if crosses is None:
        crosses = cross(triangles)
    # unitize the cross product vectors
    unit, valid = util.unitize(crosses, check_valid=True)
    return unit, valid