Example using blend layers:
You need to use "name output" to add options for input menu.
I'm not 99% sure "merge layers" does blend between input0 and input1 using input2 as blending factor in "lerp" mode, and that blending factor is threshold in "select" mode.
Btw this was actually implemented in original 1.10 and IMO this whole idea only reached prototype - very difficult to use in practice.