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In trying to answer the second question in How to determine the order of an ODE? I have unsuccessfully tried to use select to filter out the appropiate parts of the differential equations. In these attempts I have hit upon a behaviour of select I do not understand: In my opinion each of the following two code lines (or at the very least the second line) should return the differential expression itself: 
select(has,diff(y(t),t),y);
select(has,diff(y(t),t),y(t));
But they do not; each line returns diff(y(t)), where the variable t with respect to which is being differentiated is missing. Why? Is it just me having fundamentally misunderstood something?
diff select

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