Determine if the source is a definition section.

    Args:
        source: The usage string source that may be a section.

    Returns:
        True if the source describes a definition section; otherwise, False.

def _is_definition_section(source):
    """Determine if the source is a definition section.

    Args:
        source: The usage string source that may be a section.

    Returns:
        True if the source describes a definition section; otherwise, False.
    """
    try:
        definitions = textwrap.dedent(source).split('\n', 1)[1].splitlines()
        return all(
            re.match(r'\s\s+((?!\s\s).+)\s\s+.+', s) for s in definitions)
    except IndexError:
        return False