Indicate name of local variable for reads
Currently, nodes in concrete execution graphs can have labels like this:
x?0
y = $0
println($1)
This is a bit inconsistent.
I suggest we switch everything over to a notation that shows the actual value:
x?0
y!0
println(0)
If this is not clear enough, the name of the local variable could always be added in gray:
x?0 [r]
y!0 [$0]
println(0) [$1]