when deleting a wall, leave no basin without connection (connect to river upstream/downstream)
prevent deletion of walls that would be restored by the aforementioned mechanism
"del" hotkey to delete element on schema

can now create upstream to downstream direct connections
unrelevant list elements are grayed-out

grey out Calculate button if basins structure is invalid
use Mermaid Nodes for walls
suffix multiple walls on same basins pair
base Mermaid Nodes on children UIDs
display Mermaid code (debug)
unselect element after deletion
highlight selected element

add basin
remove basin (buggy)
remove wall