class_docstr: Leafs are basically tokens with a better API. Leafs exactly know where they
  were defined and what text preceeds them.
code: def get_start_pos_of_prefix(self):
      previous_leaf = self.get_previous_leaf()
      if previous_leaf is None:
          lines = split_lines(self.prefix)
          # + 1 is needed because split_lines always returns at least [''].
          return self.line - len(lines) + 1, 0  # It's the first leaf.
      return previous_leaf.end_pos
  
imports: import sys
  from abc import abstractmethod, abstractproperty
  from parso._compatibility import utf8_repr, encoding
  from parso.utils import split_lines
label_desc: Use image node_id 4 for calling the Leaf obj's underlying member method code with example usage: obj.get_start_pos_of_prefix() and returns: previous_leaf, unknown,  int