You have to install a meselamp under the following rules:
- It is always turned on
- It attached below a ceiling (any large horizontal surface will do)
- It points downwards
- Any other mesecons nodes powering the lamp must not be visible when standing directly below the meselamp and looking at it and seeing it
- All the meselamp’s neighbor nodes (including diagonals) on the X/Z plane must be air
- All the nodes which are the lower neighbors of all the nodes from rule 5 and the meselamp must be air
- It must be possible to achieve all this without relying on creative mode
- You may only use minetest_game and Mesecons
Here’s a screenshot of a meselamp which fulfufils all but the first rule:

I suppose that this seemingly simple problem is impossible to solve with the current version of Mesecons.
Reasons: First of all, the meselamp does not conceil an entire block, so you can’t just break open the block above it, as it would expose wires. In fact, you have to break open at least one block of the ceiling to even reach the lamp. But this would break rule 4, because this would make wires visible.
But even if the meselamp would conceil an entire node, it won’t work. Sure, you could now safely break open the block above it, but the meselamp is inable to receive power from above or below.
I don’t make any claims about wheather the problem is solvable if I drop the last rule. Maybe there is a mod which makes this possible. If you can find a solution when the last rule is dropped, please post it! If you can find a solution under all rules and thus prove me wrong, please post it as well!

