def _combined(expressions, operator):
    """Private method to handle expression composition with a binary infix
    operator.

    Args:
        expressions: An iterable of expressions
        operator: The binary infix operator string with which to combine the
            expressions

    Returns:
        The combined expression string.
    """
    return '({0})'.format(
        ' {0} '.format(operator).join((
            '({0})'.format(e) for e in expressions
        ))
    )