code: def _simplify_single_line(expression):
      """
      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
  
      """
      t1, t2 = extract_type_tens(expression, GammaMatrix)
      if t1 != 1:
          t1 = kahane_simplify(t1)
      res = t1 * t2
      return res
  
file_path: sympy/sympy/physics/hep/gamma_matrices.py
imports: from sympy.core.mul import Mul
  from sympy.core.singleton import S
  from sympy.matrices.dense import eye
  from sympy.matrices.expressions.trace import trace
  from sympy.tensor.tensor import TensorIndexType, TensorIndex, TensMul, TensAdd, tensor_mul, Tensor, TensorHead, TensorSymmetry
label_desc: Use image node_id 5 for calling a global function with example usage: _simplify_single_line(expression) and returns: res