function tr = Transport(r, th, model, loglevel)
% TRANSPORT  Transport class constructor.
% tr = Transport(r, th, model, loglevel)
% Create a new instance of class :mat:func:`Transport`. Three or four arguments
% may be supplied. The first two must be an instance of class:mat:func:`XML_Node`
% and an instance of class :mat:func:`ThermoPhase` respectively.
% The third argument is the type of modeling desired, specified
% by the string ``'default'``, ``'Mix'`` or ``'Multi'``.
% ``'default'`` uses the default transport specified in the
% :mat:func:`XML_Node`. The fourth argument is
% the logging level desired.
%
% :param r:
%     An instance of class :mat:func:`XML_Node`
% :param th:
%     Instance of class :mat:func:`ThermoPhase`
% :param model:
%     String indicating the transport model to use. Possible values
%     are ``'default'``, ``'Mix'``, and ``'Multi'``
% :param loglevel:
%     Level of diagnostic logging. Default if not specified is 4.
% :return:
%     Instance of class :mat:func:`Transport`
%

tr.id = 0;
if nargin == 3
    loglevel = 4;
end
if ~isa(r, 'XML_Node')
    error(['The first argument must be an instance of class XML_Node'])
elseif ~isa(th, 'ThermoPhase')
    error('The second argument must be an instance of class ThermoPhase')
else
    tr.th = th;
    if strcmp(model, 'default')
        try
            node = child(r, 'transport');
            tr.model = attrib(node, 'model');
        catch
            tr.model = 'None';
        end
    else
        tr.model = model;
    end
    tr.id = trans_get(thermo_hndl(th), -1, tr.model, loglevel) ;
    tr = class(tr, 'Transport');
end