Acnode/Examples/Policeman on Point Duty

Examples of Acnodes
Consider the locus of the equation:


 * $x^2 y^2 = a^2 \paren {x^2 + y^2}$

This is the classic quartic curve "policeman on point duty":


 * Policeman-on-point-duty.png

which has an acnode at $\tuple {0, 0}$.