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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 _einsum_equation(input_shapes, output_shape):
 """Turn shapes into an einsum equation. e.g. "ij,jk->ik" Args: input_shapes: a list of Shapes output_shape: a Shape Returns: a string """

  ret = []
  next_letter = ord("a")
  dim_to_letter = {}
  for shape_num, shape in enumerate(input_shapes + [output_shape]):
    if shape_num == len(input_shapes):
      ret.append("->")
    elif shape_num > 0:
      ret.append(",")
    for d in shape.dims:
      if d not in dim_to_letter:
        dim_to_letter[d] = chr(next_letter)
        next_letter += 1
      ret.append(dim_to_letter[d])

  return "".join(ret)