class_docstr: This class holds the broken-down components of a hole semantics, i.e. it
  extracts the holes, labels, logic formula fragments and constraints out of
  a big conjunction of such as produced by the hole semantics grammar.  It
  then provides some operations on the semantics dealing with holes, labels
  and finding legal ways to plug holes with labels.
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docstr: Return the set of labels which are not referenced directly as part of
  another formula fragment.  These will be the top-most labels for the
  subtree that they are part of.
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code: def _find_top_most_labels(self):
      """
      Return the set of labels which are not referenced directly as part of
      another formula fragment.  These will be the top-most labels for the
      subtree that they are part of.
      """
      return self._find_top_nodes(self.labels)
  
imports: from functools import reduce
  from nltk.parse import load_parser
  from nltk.sem.logic import AllExpression, AndExpression, ApplicationExpression, ExistsExpression, IffExpression, ImpExpression, LambdaExpression, NegatedExpression, OrExpression
  from nltk.sem.skolemize import skolemize
label_desc: Use image node_id 5 for calling the HoleSemantics obj's underlying member method code with example usage: obj._find_top_most_labels() and returns: self