def _get_tree_binaries(_tool):
    """
    Returns a URL where a tool's binaries can be found.
    :param _tool: Specify the tree building tool to be used.
    :return: A string containing the tool's URL.
    """
    tool_dict = {'raxml':
        'http://sco.h-its.org/exelixis/web/software/raxml/index.html',
        'phyml': 'http://www.atgc-montpellier.fr/phyml/versions.php',
        'fasttree': 'http://www.microbesonline.org/fasttree/#Install'}
    return tool_dict[_tool]
