To understand flatten() method. This is an example of hierarchy of errors resulted from validation of command_state generated by the command builder (state would be passed to created Command, if there wouldn't be errors.

Honeybee\Honeybee\AggregateRootCommandBuilder.php

    /**
     * Contains conventions for errors array format.
     * Probably used just for errors array.
     * ^.|.(not "values" or "embedded_entity_commands").(decimal|.)..
     * eg key for attribute path: .book.2.

        $errors = [
            [   // error

            ]
            'books' => [
                '@incidents' => [
                    [
                        'path' => author.books.1.title',
                        'incidents' => [ 'mandatory_missing', [ 'title', 'title_en' ] ]
                    ]
                ],
                'books.1.@type' => []
            ]
        ]
        [][$attribute_name]['@incidents'][]
======================
        

    flatten(command->build()Error) = [
        'property_name' => [
            [
                [
                    // validateEmbeddedEntityListAttribute()Result
                    [
                        'embedded_list_attribute_name' = [
                            '@incidents' = [
                                [
                                    'path' => 'embedded_list_attribute_path',
                                    'incidents' => [ 'min_count' => [ 'incident_parameters' ] ]
                                ]
                            ]
                        ]
                    ]
                ],
                [
                    // getEmbeddedCommands()Result
                    [
                        'embed_attribute_name.position.@type' => [
                            '@incidents' => [
                                [
                                    'path' => 'attribute_path',
                                    'incidents' => [
                                        [ 'incident_name' => 'incident_params']
                                    ]
                                ]
                            ]
                        ]
                    ]
                ],
                [
                    // sanitizeAttributeValue()Result
                    [
                        'attribute_name' => [
                            '@incidents' => [
                                [
                                    'path' => 'attribute_path',
                                    'incidents' => [
                                        [ 'incident_name' => 'incident_params']
                                    ]
                                ]
                            ]
                        ]
                    ]
                ],
                // attributes validation
                [
                    'attribute_name' => [
                        '@incidents' => [
                            [
                                'path' => 'attribute_path',
                                'incidents' => [
                                    [ 'incident_name' => 'incident_params']
                                ]
                            ]
                        ]
                    ]
                ],
                [
                    'title' => [
                        '@incidents' => [
                            [
                                'path' => 'author.books.book.title',
                                'incidents' => [ [ 'mandatory' => 'title'] ]
                            ]
                        ]
                    ]
                ]
            ]
        ]
    ]
     */
    protected static function flatten(array $array, $parent_prefix = '')
    {
        $flattened = [];

        foreach ($array as $key => $value) {
            $key = $parent_prefix . $key;
            $key = preg_replace('#(^\.|\.?(values|embedded_entity_commands)\.(\d|\.)?\.?)#', '', $key);
            if (is_array($value)) {
                if (isset($value['@incidents'])) {
                    $flattened[$key] = $value['@incidents'];
                } else {
                    $flattened = array_merge(self::flatten($value, $key ? $key . '.' : ''), $flattened);
                }
            } else {
                $flattened[$key] = $value;
            }
        }

        return $flattened;
    }