This method is supposed to handle all the steps needed to parse the
current subsegment of the token stream. It is supposed to return a valid
PhpCssAst.
If the parsing process can't be completed because of invalid input a
PhpCssParserException needs to be thrown.
The methods protected methods read and lookahead should be used to
operate on the token stream. They will throw PhpCssParserExceptions
automatically in case they do not succeed.
