Set: pp::colors
  Set: pp::unicode
  Assumed: a
  Assumed: b
  Assumed: c
a + (b + 0) + a
=====>
Visit, depth: 1, a + (b + 0) + a
Visit, depth: 2, Nat::add
Visit, depth: 2, a + (b + 0)
Visit, depth: 3, Nat::add
Visit, depth: 3, a
Step: a ===> a
Visit, depth: 3, b + 0
Visit, depth: 4, Nat::add
Visit, depth: 4, b
Step: b ===> b
Visit, depth: 4, 0
Rewrite using: Nat::add_zeror
  b + 0 ===> b
Step: b + 0 ===> b
Visit, depth: 3, a + b
Visit, depth: 4, Nat::add
Step: a + b ===> a + b
Step: a + (b + 0) ===> a + b
Rewrite using: Nat::add_assoc
  a + b + a ===> a + (b + a)
Visit, depth: 2, a + (b + a)
Visit, depth: 3, Nat::add
Visit, depth: 3, b + a
Visit, depth: 4, Nat::add
Rewrite using: Nat::add_comm
  b + a ===> a + b
Step: b + a ===> a + b
Visit, depth: 3, a + (a + b)
Visit, depth: 4, Nat::add
Step: a + (a + b) ===> a + (a + b)
Step: a + (b + a) ===> a + (a + b)
Step: a + (b + 0) + a ===> a + (a + b)
a + (a + b)
trans (trans (congr1 a (congr2 Nat::add (congr2 (Nat::add a) (Nat::add_zeror b)))) (Nat::add_assoc a b a))
      (congr2 (Nat::add a) (Nat::add_comm b a))