Strolling around I came across Craig Gidney's Blog and his awesome tool Quirk. I proceeded to read this interesting post of his where he alludes to the difference between actually measuring qubits in circuits and simply conditioning some part of the circuit to state of qubits. Long story short, he provides this Quirk circuit to experiment with. There he has a gate that is called "x499" and my question is:
How do I make this gate or any other of the input gates? (Those that feature an "A" or "B") I have spent a good amount of time dragging and dropping and clicking around but I seem unable to figure this functionality out. (* ashamed *) If I drag it into my circuit, it will turn yellow and show something like "input A needed".