class_docstr: References
  ==========
  
  .. [1] Holt, D., Eick, B., O'Brien, E.
         "Handbook of Computational Group Theory"
         Section 8.1.3
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docstr: Return the leading non-zero exponent.
  
  Explanation
  ===========
  
  The leading exponent for a given element `g` is defined
  by `\mathrm{leading\_exponent}[g]` `= e_i`, if `\mathrm{depth}[g] = i`.
  
  Examples
  ========
  
  >>> from sympy.combinatorics.named_groups import SymmetricGroup
  >>> G = SymmetricGroup(3)
  >>> PcGroup = G.polycyclic_group()
  >>> collector = PcGroup.collector
  >>> collector.leading_exponent(G[1])
  1
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code: def leading_exponent(self, element):
      r"""
      Return the leading non-zero exponent.
  
      Explanation
      ===========
  
      The leading exponent for a given element `g` is defined
      by `\mathrm{leading\_exponent}[g]` `= e_i`, if `\mathrm{depth}[g] = i`.
  
      Examples
      ========
  
      >>> from sympy.combinatorics.named_groups import SymmetricGroup
      >>> G = SymmetricGroup(3)
      >>> PcGroup = G.polycyclic_group()
      >>> collector = PcGroup.collector
      >>> collector.leading_exponent(G[1])
      1
  
      """
      exp_vector = self.exponent_vector(element)
      depth = self.depth(element)
      if depth != len(self.pcgs) + 1:
          return exp_vector[depth - 1]
      return None
  
imports: from sympy.ntheory.primetest import isprime
  from sympy.combinatorics.perm_groups import PermutationGroup
  from sympy.printing.defaults import DefaultPrinting
  from sympy.combinatorics.free_groups import free_group
label_desc: Use image node_id 10 for calling the Collector obj's underlying member method code with example usage: obj.leading_exponent(element) and returns: None, exp_vector