class_docstr: Add Prefix to src (& tgt) sentence.
docstr: Prepend `prefix` to `tokens`.
docstr_tok: ["Prepend","`","prefix","`","to","`","tokens","`","."]
code: def _prepend(self, example, prefix):
      """Prepend `prefix` to `tokens`."""
      for side, side_prefix in prefix.items():
          if example.get(side) is not None:
              example[side] = side_prefix.split() + example[side]
          elif len(side_prefix) > 0:
              example[side] = side_prefix.split()
      return example
  
imports: from onmt.utils.logging import logger
  from onmt.transforms import register_transform
  from .transform import Transform, ObservableStats
label_desc: Use image node_id 7 for calling the PrefixTransform obj's underlying member method code with example usage: obj._prepend(example, prefix) and returns: example