www_browser.pl -- Open a URL in the users browser

This library deals with the highly platform specific task of opening a web page. In addition, is provides a mechanism similar to absolute_file_name/3 that expands compound terms to concrete URLs. For example, the SWI-Prolog home page can be opened using:

?- www_open_url(swipl(.)).

www_open_url(+Url)

Open URL in running version of the users' browser or start a new browser. This predicate tries the following steps:

If a prolog flag (see set_prolog_flag/2) browser is set and this is the name of a known executable, use this. The flag may be set to Command-Mode, where mode is one of fg or bg, requesting Command to run in foreground or background mode. Default is bg.
On Windows, use win_shell(open, URL)
Find a generic `open' comment. Candidates are xdg-open, open or gnome-open.
If a environment variable BROWSER is set and this is the name of a known executable, use this.
Try to find a known browser.
@tbd Figure out the right tool in step 3 as it is not uncommon that multiple are installed.

known_browser(+FileBaseName, -Compatible)[multifile]

True if browser FileBaseName has a remote protocol compatible to Compatible.

expand_url_path(+Spec, -URL)

Expand URL specifications similar to absolute_file_name/3. The predicate url_path/2 plays the role of file_search_path/2.

Errors: - existence_error(url_path, Spec) if the location is not defined.