class_docstr: This class will be used by a dictionary that will store information
  about predicates to be used by the ``ClosedWorldProver``.
  
  The 'signatures' property is a list of tuples defining signatures for
  which the predicate is true.  For instance, 'see(john, mary)' would be
  result in the signature '(john,mary)' for 'see'.
  
  The second element of the pair is a list of pairs such that the first
  element of the pair is a tuple of variables and the second element is an
  expression of those variables that makes the predicate true.  For instance,
  'all x.all y.(see(x,y) -> know(x,y))' would result in "((x,y),('see(x,y)'))"
  for 'know'.
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code: def __repr__(self):
      return "%s" % self
  
imports: from collections import defaultdict
  from functools import reduce
  from nltk.inference.api import Prover, ProverCommandDecorator
  from nltk.inference.prover9 import Prover9, Prover9Command
  from nltk.sem.logic import AbstractVariableExpression, AllExpression, AndExpression, ApplicationExpression, BooleanExpression, EqualityExpression, ExistsExpression, Expression, ImpExpression, NegatedExpression, Variable, VariableExpression, operator, unique_variable
label_desc: Use image node_id 6 for calling the PredHolder obj's underlying member method code with example usage: obj.__repr__() and returns: unknown