Back in the day, I had used the voltage-controlled switch component, 'S',  in MicroSim PSpice (the student version).

Thinking I would use it in QucsStudio, I made an equation-based subcircuit for a switch.  In the process, I came across the equation for the switch used in LTspice.  

The slides below show subcircuits for both.  In the end, I did not use the switches, but have put them here in case my needs change.