Panel ToggleButton Passthrough

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I cannot figure out how to provide the correct pt object to customize the ToggleButton properties in the Panel component.

I wish to modify the ToggleButton offIcon and onIcon properties, to display alternate glyphs than the default plus/minus glyphs.

I have tried variations on this to no avail:

:pt="{pcToggleButton: { props: { offIcon:'pi pi-angle-up', onIcon: 'pi pi-angle-down'}}}"
:toggleButtonProps="{offIcon: 'pi pi-angle-up', onIcon: 'pi pi-angle-down'}"

It is totally unclear when to insert a root or other intermediate property; the documentation for using passthrough needs more detailed examples, or each component needs some specific examples for configuring each PT component.

I have also tried the ptOptions and toggleButtonProps props to no avail.

Can someone please demonstrate what is the correct object for this, using pt, ptOptions or toggleButtonProps?