<?xml version="1.0" ?>
<sdf version='1.7'>
  <world name="world_frame_attached_to">

    <frame name="world_frame"/>          <!-- VALID: Directly attached_to implicit world frame. -->
    <frame name="F00" attached_to="world"/> <!-- VALID: Directly attached_to implicit world frame. -->
    <frame name="F0" attached_to=""/>    <!-- VALID: Directly attached_to implicit world frame by default. -->
    <frame name="F1" attached_to="F0"/>  <!-- VALID: Indirectly attached_to implicit world frame via frame F0. -->

    <model name="M1">
      <link name="L"/>
      <frame name="F0" attached_to="L"/> <!-- VALID: Directly attached_to link L. -->
    </model>

    <frame name="F2" attached_to="M1"/>  <!-- VALID: Indirectly attached to canonical link L of model M1. -->
  </world>
</sdf>
