code: def get_branch_length(self, age=None, pos=0):
          
          if age is None:
              age = self.age
  
          return self.length * pow(self.branches[pos][0], age)
docs: Get the length of a branch.
  
          This method calculates the length of a branch in specific age.
          The used formula: length * scale^age.
  
          Args:
              age (int): The age, for which you want to know the branch length.
          Returns:
              float: The length of the branch