def wavedec2_keys(level):
    """Subband keys corresponding to a wavedec2 decomposition."""
    approx = ''
    coeffs = {}
    for lev in range(level):
        for k in ['a', 'h', 'v', 'd']:
            coeffs[approx + k] = None
        approx = 'a' * (lev + 1)
        if lev < level - 1:
            coeffs.pop(approx)
    return list(coeffs.keys())