Parse an input string, and return an AST
        doctype must have WCADocument as a baseclass

def parse(self, data, doctype):
        '''
        Parse an input string, and return an AST
        doctype must have WCADocument as a baseclass
        '''
        self.doctype = doctype
        self.lexer.lineno = 0
        del self.errors[:]
        del self.warnings[:]
        self.lexer.lexerror = False
        ast = self.parser.parse(data, lexer=self.lexer)
        if self.lexer.lexerror:
            ast = None
        if ast is None:
            self.errors.append("Couldn't build AST.")
        else:
            for check in self.sema[self.doctype]:
                visitor = check()
                if not visitor.visit(ast):
                    self.errors.append("Couldn't visit AST.")
                self.errors.extend(visitor.errors)
                self.warnings.extend(visitor.warnings)
        return (ast, list(self.errors), list(self.warnings))