class_docstr: Add Prefix to src (& tgt) sentence.
docstr: Get special vocabs added by prefix transform.
docstr_tok: ["Get","special","vocabs","added","by","prefix","transform","."]
code: def get_specials(cls, opts):
      """Get special vocabs added by prefix transform."""
      prefix_dict = cls.get_prefix_dict(opts)
      src_specials, tgt_specials = set(), set()
      for _, prefix in prefix_dict.items():
          src_specials.update(prefix["src"].split())
          tgt_specials.update(prefix["tgt"].split())
      return (src_specials, tgt_specials)
  
imports: from onmt.utils.logging import logger
  from onmt.transforms import register_transform
  from .transform import Transform, ObservableStats
label_desc: Use image node_id 5 for calling the PrefixTransform obj's underlying member method code with example usage: obj.get_specials(cls, opts) and returns: src_specials,  tgt_specials