I recently came across this very nice question in a textbook: 

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> Find the possible values of the parameter $c$ for which $$4x^2 +(c-1)^2y^2 +2cx+6y=0$$ represents a circle. 
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The solution was easy enough, but it got me wondering about whether a similar equation involving a parameter could be found that could represent EITHER a circle OR a pair of straight lines in the $(x,y)$ plane depending on how we choose $c$? Can anyone think of such an equation?