Return the coherent state written out as an indexed sum over Fock
        basis states

def to_fock_representation(self, index_symbol='n', max_terms=None):
        """Return the coherent state written out as an indexed sum over Fock
        basis states"""
        phase_factor = sympy.exp(
            sympy.Rational(-1, 2) * self.ampl * self.ampl.conjugate())
        if not isinstance(index_symbol, IdxSym):
            index_symbol = IdxSym(index_symbol)
        n = index_symbol
        if max_terms is None:
            index_range = IndexOverFockSpace(n, hs=self._hs)
        else:
            index_range = IndexOverRange(n, 0, max_terms-1)
        term = (
            (self.ampl**n / sympy.sqrt(sympy.factorial(n))) *
            BasisKet(FockIndex(n), hs=self._hs))
        return phase_factor * KetIndexedSum(term, index_range)