import data.nat
open nat tactic

example (a b : nat) : a * 0 + 0 + b + 0 = b :=
by do
 rewrite `mul_zero,
 trace_state,
 rewrite `add_zero,
 repeat $ rewrite `zero_add

print "---------"

example (a b : nat) (H : a + b * 0 + 0 = b) : b = a :=
by do
 rewrite_at `mul_zero `H,
 trace_state,
 rewrite_at `add_zero `H,
 rewrite_at `add_zero `H,
 symmetry,
 assumption

print "---------"

example (a : nat) : (0 + a) + (0 + a) + (0 + a) = a + a + a :=
by rewrite `zero_add

meta_definition rewrite_occs (th_name : name) (occs : list nat) : tactic unit :=
do th ← mk_const th_name,
   rewrite_core reducible tt (occurrences.pos occs) ff th,
   try reflexivity

print "---------"

example (a : nat) : (0 + a) + (0 + a) + (0 + a) = a + a + a :=
by do
   rewrite_occs `zero_add [1, 3],
   trace_state,
   rewrite `zero_add

print "---------"

example (a : nat) : (0 + a) + (0 + a) + (0 + a) = a + a + a :=
by do
   rewrite_occs `zero_add [2],
   trace_state,
   rewrite_occs `zero_add [2],
   trace_state,
   rewrite `zero_add