docs: def __parse_relation():
      '''Gets and parses file'''
      relation_filename = get_file('relation.tsv')
      vertice_filename = get_file('vertice.tsv')
      relation_textfile = open(relation_filename, 'r')
      vertice_textfile = open(vertice_filename, 'r')
  
      # Parse vertice:
      vertices = {}
  
      next(vertice_textfile)
  
      for line in vertice_textfile:
          tokens = line.strip().split('\t')
          vertices[tokens[0]] = tokens[1]
  
      next(relation_textfile)
  
      for line in relation_textfile:
          tokens = line.strip().split('\t')
  
          source_chebi_id = int(vertices[tokens[3]])
          target_chebi_id = int(vertices[tokens[2]])
          typ = tokens[1]
  
          if source_chebi_id not in __OUTGOINGS:
              __OUTGOINGS[source_chebi_id] = []
  
          if target_chebi_id not in __INCOMINGS:
              __INCOMINGS[target_chebi_id] = []
  
          target_relation = Relation(typ, str(target_chebi_id), tokens[4])
          source_relation = Relation(typ, str(source_chebi_id), tokens[4])
  
          __OUTGOINGS[source_chebi_id].append(target_relation)
          __INCOMINGS[target_chebi_id].append(source_relation)