Default emit method: visit both ASTs and return the codegen

def emit(self, ast_reg, ast_guide):
        ''' Default emit method: visit both ASTs and return the codegen '''
        if (ast_reg):
            self.visit(ast_reg)
        codegen_reg = self.codegen
        self.codegen = self.cg_type()
        if (ast_guide):
            self.visit(ast_guide)
        return (codegen_reg, self.codegen)