code: def from_irafpath(irafpath):
      
      s = irafpath.split('$')
  
      if len(s) != 2:
          return irafpath
      if len(s[0]) == 0:
          return irafpath
  
      try:
          refdir = os.environ[s[0]]
      except KeyError:
          raise ValueError('{0} environment variable undefined'.format(s[0]))
  
      return os.path.join(refdir, s[1])
docs: Resolve IRAF path like ``jref$`` into actual file path.
  
      Parameters
      ----------
      irafpath : str
          Path containing IRAF syntax.
  
      Returns
      -------
      realpath : str
          Actual file path. If input does not follow ``path$filename``
          format, then this is the same as input.
  
      Raises
      ------
      ValueError
          The required environment variable is undefined.