'get the commutation group number corresponding to ``i``
``i`` can be a symbol or a number or a string
If ``i`` is not already defined its commutation group number
is set.'

def comm_symbols2i(self, i):

    if (i not in self._comm_symbols2i):
        n = len(self._comm)
        self._comm.append({})
        self._comm[n][0] = 0
        self._comm[0][n] = 0
        self._comm_symbols2i[i] = n
        self._comm_i2symbol[n] = i
        return n
    return self._comm_symbols2i[i]
