  
  
  [1XReferences[0X
  
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  [[20XHig76[15X]  [16XHiggins,  P.[15X, [17XThe fundamental groupoid of a graph of groups[15X, [18XJ.
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  [[20XMoo01[15X]  [16XMoore, E.[15X, [17XGraphs of groups: word computations and free crossed
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