Blender addon UI create collapsible subpanel

subPanel.py

import bpy

class EXAMPLE_panel:
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_category = "Hi"
    bl_options = {"DEFAULT_CLOSED"}

#The primary panel, no parents
class EXAMPLE_PT_panel_1(EXAMPLE_panel, bpy.types.Panel):
    bl_idname = "EXAMPLE_PT_panel_1"
    bl_label = "Main"

    def draw(self, context):
        pass

#subpanel only used as a grouping of child panels
class EXAMPLE_PT_panel_2(EXAMPLE_panel, bpy.types.Panel):
    bl_parent_id = "EXAMPLE_PT_panel_1"
    bl_label = "Subpanel"

    def draw(self, context):
        layout = self.layout
        layout.label(text="Here there will be more stuff")

#child panel, without showing title
class EXAMPLE_PT_panel_3(EXAMPLE_panel, bpy.types.Panel):
    bl_parent_id = "EXAMPLE_PT_panel_1"
    bl_label = "(should not exist)"
    bl_options = {'HIDE_HEADER'}

    def draw(self, context):
        column = self.layout.column(align = True)
        column.operator("mesh.primitive_plane_add", text = "Plane") 
        
classes = (EXAMPLE_PT_panel_1, EXAMPLE_PT_panel_2, EXAMPLE_PT_panel_3)

def register():
    for cls in classes:
        bpy.utils.register_class(cls)

def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)

if __name__ == "__main__":
    register()