renderToString = function(expression, options) {
                    var settings = new Settings(options);

                    var tree = parseTree(expression, settings);
                    return buildTree(tree, expression, settings).toMarkup();
                }
Gets the final math type of an expression, given its group type. This type is
used to determine spacing between elements, and affects bin elements by
causing them to change depending on what types are around them. This type
must be attached to the outermost node of an element as a CSS class so that
spacing with its surrounding elements works correctly.

Some elements can be mapped one-to-one from group type to math type, and
those are listed in the `groupToType` table.

Others (usually elements that wrap around other elements) often have
recursive definitions, and thus call `getTypeOfGroup` on their inner
elements.