def start(self, labels=None):
        """Start specified timer(s).

        Parameters
        ----------
        labels : string or list, optional (default None)
          Specify the label(s) of the timer(s) to be started. If it is
          ``None``, start the default timer with label specified by the
          ``dfltlbl`` parameter of :meth:`__init__`.
        """

        # Default label is self.dfltlbl
        if labels is None:
            labels = self.dfltlbl
        # If label is not a list or tuple, create a singleton list
        # containing it
        if not isinstance(labels, (list, tuple)):
            labels = [labels,]
        # Iterate over specified label(s)
        t = timer()
        for lbl in labels:
            # On first call to start for a label, set its accumulator to zero
            if lbl not in self.td:
                self.td[lbl] = 0.0
                self.t0[lbl] = None
            # Record the time at which start was called for this lbl if
            # it isn't already running
            if self.t0[lbl] is None:
                self.t0[lbl] = t
Start specified timer(s).

        Parameters
        ----------
        labels : string or list, optional (default None)
          Specify the label(s) of the timer(s) to be started. If it is
          ``None``, start the default timer with label specified by the
          ``dfltlbl`` parameter of :meth:`__init__`.