On hover/focus, blocks should draw lines between function definitions and uses

When I hover over a variable block, I want to see a line from that block to where it's defined (i.e., the variable definition, or the list comprehension block, or the function definition, which is responsible for its existence). Conversely, when I hover over one of those blocks, I want to see lines to ALL of the occurrences of that variable elsewhere in the program.

This should also work for selecting the block, since hovering isn't possible on touch screen devices.