code: def _append_first(first, rest):
      if first is None:
          return rest
      return tf.concat((tf.expand_dims(first, axis=-1), rest), axis=-1)
  
file_path: tf-quant-finance/tf_quant_finance/math/pde/steppers/parabolic_equation_stepper.py
imports: import tensorflow.compat.v2
  from tf_quant_finance import utils
label_desc: Use image node_id 12 for calling a global function with example usage: _append_first(first, rest) and returns: tf, rest