def complementary(sequence):
    """Return complementary sequence.

    Examples
    --------
    >>> complementary('AT CG')
    'CGAT'

    """
    c = {'A': 'T', 'T': 'A', 'C': 'G', 'G': 'C'}
    return ''.join(c.get(nucleotide, '') for nucleotide in reversed(sequence))