docs: def _writeBk(target="sentenceContainsTarget(+SID,+WID).", treeDepth="3",
               nodeSize="3", numOfClauses="8"):
      """
      Writes a background file to disk.
  
      :param target: Target predicate with modes.
      :type target: str.
      :param treeDepth: Depth of the tree.
      :type treeDepth: str.
      :param nodeSize: Maximum size of each node in the tree.
      :type nodeSize: str.
      :param numOfClauses: Number of clauses in total.
      :type numOfClauses: str.
      """
  
      with open('bk.txt', 'w') as bk:
  
          bk.write("useStdLogicVariables: true\n")
  
          bk.write("setParam: treeDepth=" + str(treeDepth) + '.\n')
          bk.write("setParam: nodeSize=" + str(nodeSize) + '.\n')
          bk.write("setParam: numOfClauses=" + str(numOfClauses) + '.\n')
  
          bk.write("mode: nextSentenceInBlock(+BID,+SID,-SID).\n")
          bk.write("mode: nextSentenceInBlock(+BID,-SID,+SID).\n")
          bk.write("mode: earlySentenceInBlock(+BID,-SID).\n")
          bk.write("mode: midWaySentenceInBlock(+BID,-SID).\n")
          bk.write("mode: lateSentenceInBlock(+BID,-SID).\n")
          bk.write("mode: sentenceInBlock(-SID,+BID).\n")
          bk.write("mode: wordString(+WID,#WSTR).\n")
          bk.write("mode: partOfSpeechTag(+WID,#WPOS).\n")
          bk.write("mode: nextWordInSentence(+SID,+WID,-WID).\n")
          bk.write("mode: earlyWordInSentence(+SID,-WID).\n")
          bk.write("mode: midWayWordInSentence(+SID,-WID).\n")
          bk.write("mode: lateWordInSentence(+SID,-WID).\n")
          bk.write("mode: wordInSentence(-WID,+SID).\n")
  
          bk.write("mode: " + target + "\n")
  
      return