def _simplify_single_line(expression):
    "\n    Simplify single-line product of gamma matrices.\n\n    Examples\n    ========\n\n    >>> from sympy.physics.hep.gamma_matrices import GammaMatrix as G,         LorentzIndex, _simplify_single_line\n    >>> from sympy.tensor.tensor import tensor_indices, TensorHead\n    >>> p = TensorHead('p', [LorentzIndex])\n    >>> i0,i1 = tensor_indices('i0:2', LorentzIndex)\n    >>> _simplify_single_line(G(i0)*G(i1)*p(-i1)*G(-i0)) + 2*G(i0)*p(-i0)\n    0\n\n    "
    (t1, t2) = extract_type_tens(expression, GammaMatrix)
    if (t1 != 1):
        t1 = kahane_simplify(t1)
    res = (t1 * t2)
    return res
Simplify single-line product of gamma matrices.

Examples
========

>>> from sympy.physics.hep.gamma_matrices import GammaMatrix as G,         LorentzIndex, _simplify_single_line
>>> from sympy.tensor.tensor import tensor_indices, TensorHead
>>> p = TensorHead('p', [LorentzIndex])
>>> i0,i1 = tensor_indices('i0:2', LorentzIndex)
>>> _simplify_single_line(G(i0)*G(i1)*p(-i1)*G(-i0)) + 2*G(i0)*p(-i0)
0
def _simplify_single_line(expression):
    "\n    Simplify single-line product of gamma matrices.\n\n    Examples\n    ========\n\n    >>> from sympy.physics.hep.gamma_matrices import GammaMatrix as G,         LorentzIndex, _simplify_single_line\n    >>> from sympy.tensor.tensor import tensor_indices, TensorHead\n    >>> p = TensorHead('p', [LorentzIndex])\n    >>> i0,i1 = tensor_indices('i0:2', LorentzIndex)\n    >>> _simplify_single_line(G(i0)*G(i1)*p(-i1)*G(-i0)) + 2*G(i0)*p(-i0)\n    0\n\n    "
    (t1, t2) = extract_type_tens(expression, GammaMatrix)
    if (t1 != 1):
        t1 = kahane_simplify(t1)
    res = (t1 * t2)
    return res