This function returns information about placement of a child dock object inside another dock object.
The function returns true if child is effectively a child of this.
placement should normally be initially setup to NONE. If it's set to
some other value, this function will not touch the stored value if the specified placement is "compatible" with the actual placement of
the child.
placement can be null, in which case the function simply tells if child is attached to
this.
| this |
the dock object we are asking for child placement |
| child |
the child of the this we want the placement for |
| placement |
where to return the placement information |
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