sel_id() - Select an object by its id, e.g.: sel_id('btn') selects shiny::actionButton('btn', 'Button').
sel_all() - Selects everything.
sel_input() - Selects an input by its id, e.g.: sel_id('txt') selects shiny::textInput('txt', 'Text').
sel_class() - Select all elements bearing a specific class, e.g.: sel_class('cls'), selects shiny::h1('hello', class = 'cls').
sel_tag() - Select all tags, e.g.: sel_tag('p') selects p('hello').
sel_attr() - Select all tags with a specific attribute.