Why is my plugin not triggered?

I'm working on a plugin which should provide support for plugins. But I don't get it to call (and execute) its own plugins - and I have read the doc forward and backward and for the life of me am not seeing the problem.

So I build a simple example to understand the mechanism - and fortunately the example fails as well, so at least my (mis)-understanding is consistent...

The main-plugin is this (mainplugin.php).

<?php

jimport('joomla.plugin.plugin');

class plgContentMainplugin extends JPlugin
{
    function onAfterRender()
    {                           
        JPluginHelper::importPlugin('mainplugin_plugin');
        $dispatcher = JDispatcher::getInstance();
        $a="collecting calls\n";
        $data = array("Hello", &$a);
        $res = $dispatcher->trigger('onmainiscalling', $data);
        $tmp="res of trigger: " . nl2br(print_r($res,true));
        echo '<div style="border: 3px black solid; background-color: yellow; color: black;">';
        echo '<h1>Here is the result of "onmainiscalling"</h1>' . $tmp;
        echo '</div>';
    }

    function onmainiscalling($data) {
        $r="own fn was called";
        return $r;
    }
}

?>

And it is installed with mainplugin.xml:

<?xml version="1.0" encoding="utf-8"?>
<extension type="plugin" version="1.6" method="upgrade" group="content">
    <name>Content - mainplugin</name>
    <creationDate>2013-08-07</creationDate>
    <version>1</version>
    <releaseDate>2013-08-07 22:00:21</releaseDate>
    <releaseType>First public release!</releaseType>
    <author>Michael Baas</author>
    <authorEmail>mb@mbaas.de</authorEmail>
    <authorUrl>mbaas.de</authorUrl>
    <copyright>(c) 2005-2013 Michael Baas</copyright>
    <description>Testing the plugins-for-plugins-concept</description>
    <license>free free free</license>
    <files>
        <filename plugin="mainplugin">mainplugin.php</filename>
    </files>
    <config />
</extension>

Here is a plugin for mainplugin of type mainplugin_plugin and it is called mainplugin_plugin.php:

<?php

class plgContentMainplugin_plugin extends  plgContentMainplugin // or should it be JPlugin? Tried both, nothing worked.
{

    function onmainiscalling($data) {
        $r="Mainplugin_plugin->onmainiscalling was called";
        return $r;
    }

}

?>

and its installer (mainplugin_plugin.xml):

<?xml version="1.0" encoding="utf-8"?>
<extension type="plugin" version="1.6" method="upgrade" group="mainplugin_plugin">
    <name>Content - mainplugin_plugin</name>
    <creationDate>2013-08-07</creationDate>
    <version>1</version>
    <releaseDate>2013-08-07 22:00:21</releaseDate>
    <releaseType>First public release!</releaseType>
    <author>Michael Baas</author>
    <authorEmail>mb@mbaas.de</authorEmail>
    <authorUrl>mbaas.de</authorUrl>
    <copyright>(c) 2005-2013 Michael Baas</copyright>
    <description>Testing the plugins-for-plugins-concept</description>
    <license>free free free</license>
    <files>
        <filename plugin="mainplugin_plugin">mainplugin_plugin.php</filename>
    </files>
    <config />
</extension>

In your xml you say the group is

group="mainplugin_plugin"

    JPluginHelper::importPlugin('mainplugin_plugin');

but then you are treating it like part of the content group later.

class plgContentMainplugin_plugin extends  plgContentMainplugin

You need to make up your mind on this and make sure the plugin is in the right folder and you are calling by the right groupt name.

Note that importPlugin takes the group name as the first argument and the specific plugin name as the second optional one.

I also don't know why you would be extending a plugin class normally. I mean you could ... but I don't think if you do that you should be extending it in a different group.