class_docstr: A simple balanced binary order statistic tree to help compute the concordance.
  
  When computing the concordance, we know all the values the tree will ever contain. That
  condition simplifies this tree a lot. It means that instead of crazy AVL/red-black shenanigans
  we can simply do the following:
  
  - Store the final tree in flattened form in an array (so node i's children are 2i+1, 2i+2)
  - Additionally, store the current size of each subtree in another array with the same indices
  - To insert a value, just find its index, increment the size of the subtree at that index and
    propagate
  - To get the rank of an element, you add up a bunch of subtree counts
class_docstr_tok: ["A","simple","balanced","binary","order","statistic","tree","to","help","compute","the","concordance",".","When","computing","the","concordance",",","we","know","all","the","values","the","tree","will","ever","contain",".","That","condition","simplifies","this","tree","a","lot",".","It","means","that","instead","of","crazy","AVL\/red-black","shenanigans","we","can","simply","do","the","following",":","-","Store","the","final","tree","in","flattened","form","in","an","array","(","so","node","i","'s","children","are","2i+1",",","2i+2",")","-","Additionally",",","store","the","current","size","of","each","subtree","in","another","array","with","the","same","indices","-","To","insert","a","value",",","just","find","its","index",",","increment","the","size","of","the","subtree","at","that","index","and","propagate","-","To","get","the","rank","of","an","element",",","you","add","up","a","bunch","of","subtree","counts"]
code: def __len__(self):
      return self._counts[0]
  
imports: import numpy
label_desc: Use image node_id 4 for calling the _BTree obj's underlying member method code with example usage: obj.__len__() and returns: self