Access to the poses in poselib

I cannot solve one problem that I stuck with.
I have an object with assigned armature (3 bones). To this armature is assigned poselib, with several poses.
Now my intention is to export bones’ data for each pose to the external text file (location/rotation)
As far I am able to export these data to the current pose via. bpy.context.selected_objects[0].pose.bones, but how to iterate trough all poses defined in poselib?
I will be very grateful for any hint.