Use arrow keys to move the player, also press down arrow to crouch. Press and hold S to place the ground. Press C to clear. Press N to switch building type. Press G to float and F to switch back to normal gravity. You can even double-jump, and drag the player.
Credit to @griffpatch_tutor for the tutorial, @griffpatch for the costumes. I made the dragging costume and no-gravity fall costume by myself. Gravity variable is now public!
