- show backtrace:

  2 function Foo[2]
  1 Bar[2]
->0 Bazz
line 2: let var3 = "another var"

show variables on different levels:

6
  2 function Foo[2]
->1 Bar[2]
  0 Bazz
line 2: let var3 = "another var"
3
->2 function Foo[2]
  1 Bar[2]
  0 Bazz
line 2: let var3 = "another var"
1

- undefined vars:

undefined var3 on former level:
Error detected while processing function Foo[2]..Bar[2]..Bazz:
line    3:
E121: Undefined variable: var3
here var3 is defined with "another var":
another var

undefined var2 on former level
Error detected while processing function Foo[2]..Bar:
line    3:
E121: Undefined variable: var2
here var2 is defined with 10:
10

- backtrace movements:

  1 function Foo[2]
->0 Bar
line 3: End of function

next command cannot go down, we are on bottom

frame is zero

next command cannot go up, we are on top

frame at highest level: 1
->1 function Foo[2]
  0 Bar
line 3: End of function
fil is not frame or finish, it is file
"[No Name]" --No lines in buffer--

- relative backtrace movement

  1 function Foo[2]
->0 Bar
line 3: End of function
->1 function Foo[2]
  0 Bar
line 3: End of function

- go beyond limits does not crash

frame at highest level: 1
->1 function Foo[2]
  0 Bar
line 3: End of function
frame is zero
  1 function Foo[2]
->0 Bar
line 3: End of function

- final result 19:
19