def parse_prefixes(root):
    """Returns a dictionary of unit of measure prefixes."""

    prefixes = {}
    for node in root.find("{*}prefixSet"):
        name = node.find('{*}name').text
        prefixes[name] = dict(
            symbol = node.find('{*}symbol').text,
            multiplier = node.find('{*}multiplier').text,
        )
        common_name = node.find('{*}commonName')
        if common_name is not None:
            prefixes[name]['common_name'] = common_name.text

    return prefixes