ADDED: new commands for managing numbered variables
* sweeprolog.el (sweeprolog-increment-numbered-variables)
(sweeprolog-decrement-numbered-variables): new commands.
(sweeprolog-mode-map): bind them to C-c C-+ and C-c C--.
(sweeprolog-context-menu-for-variable): allow calling these new
commands from context menus for numbered variables.
* sweeprolog-tests.el: add test for incrementing numbered variables.
* README.org (Numbered Variables): new manual section.