how to build form

1，drupal_get_form($form_id)函数中调用drupal_retrieve_form函数。
2，drupal_retrieve_form()调用构造form元素树的函数，得到$form变量实例

5, 核心的环节
drupal_render函数调用。
drupal_render($form);
call theme() // $content = theme($elements['#theme'], $elements);

call init_theme()
call list_themes()
call _init_theme()
call _theme_load_registry