class_docstr: Abstract class for representing a binary combinator.
  Merely defines functions for checking if the function and argument
  are able to be combined, and what the resulting category is.
  
  Note that as no assumptions are made as to direction, the unrestricted
  combinators can perform all backward, forward and crossed variations
  of the combinators; these restrictions must be added in the rule
  class.
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code: def combine(self, function, argument):
      pass
  
imports: from abc import ABCMeta, abstractmethod
  from nltk.ccg.api import FunctionalCategory
label_desc: Use image node_id 2 for calling the UndirectedBinaryCombinator obj's underlying member method code with example usage: obj.combine(function, argument) without return types