parseTree = function(toParse, settings) {
                    var parser = new Parser(toParse, settings);

                    return parser.parse();
                }
Rewrites infix operators such as \over with corresponding commands such
as \frac.

There can only be one infix operator per group.  If there's more than one
then the expression is ambiguous.  This can be resolved by adding {}.

@returns {Array}