%helps global variable: paths to help directories of loaded external modules

## Description

The global variable `%helps` is an N x 2 matrix of strings. Each row refers to the help chapter of a loaded external module:

- `%helps(k,1)` is the absolute pathname to its help directory.
- `%helps(k,2)` is the title of its help chapter.

This variable must NOT be modified by hand. `add_help_chapter` must be used to add a new help chapter and update `%helps`.

## Examples

    global %helps  // This instruction creates a local %helps variable connected to the global one
    %helps
    if %helps==[]                    // No help chapter is registered for any external module
        if atomsGetInstalled()==[]   // If no external module is installed
            atomsInstall uman;       //  we install the "uman" one (available for all plateforms)
        end
        atomsLoad(atomsGetInstalled()(1));  // We load the first module, included its help pages
        %helps                       // Now the external chapter should be registered
    end

## See also

- add_help_chapter
- help