meta def basic_monitor : vm_monitor nat :=
{ init := 0, step := λ s, return (trace ("step " ++ s^.to_string)  (λ u, s+1)) >> failure }

run_command vm_monitor.register `basic_monitor

set_option debugger true

def f : nat → nat
| 0     := 0
| (a+1) := f a

vm_eval trace "a" (λ u, f 4)