def _adjacent(imp1, imp2):
    """Determine if two implicants are adjacent: ie differ on only one variable.

    Args:
        imp1 (string): implicant 1
        imp1 (string): implicant 2

    Returns:
        (bool)
    """
    differences = 0
    match = []
    for m1, m2 in zip(imp1, imp2):
        if m1 == m2:
            match.append(m1)
        elif differences:
            return False
        else:
            differences += 1
            match.append('2')
    return "".join(match)