-- Mathlib/NumberTheory/Padics/PadicVal/Basic.lean
theorem le_padicValRat_add_of_le {q r : ℚ} (hqr : q + r ≠ 0)
    (h : padicValRat p q ≤ padicValRat p r) : padicValRat p q ≤ padicValRat p (q + r) :=
  if hq : q = 0 then by simpa [hq] using h
  else
    if hr : r = 0 then by simp [hr]
    else by
      have hqn : q.num ≠ 0 := Rat.num_ne_zero.2 hq
      have hqd : (q.den : ℤ) ≠ 0 := mod_cast Rat.den_nz _
      have hrn : r.num ≠ 0 := Rat.num_ne_zero.2 hr
      have hrd : (r.den : ℤ) ≠ 0 := mod_cast Rat.den_nz _
      have hqreq : q + r = (q.num * r.den + q.den * r.num) /. (q.den * r.den) := Rat.add_num_den _ _
      have hqrd : q.num * r.den + q.den * r.num ≠ 0 := Rat.mk_num_ne_zero_of_ne_zero hqr hqreq
      conv_lhs => rw [← q.num_divInt_den]
      rw [hqreq, padicValRat_le_padicValRat_iff hqn hqrd hqd (mul_ne_zero hqd hrd), ←
        emultiplicity_le_emultiplicity_iff, mul_left_comm,
        emultiplicity_mul (Nat.prime_iff_prime_int.1 hp.1), add_mul]
      rw [← q.num_divInt_den, ← r.num_divInt_den, padicValRat_le_padicValRat_iff hqn hrn hqd hrd, ←
        emultiplicity_le_emultiplicity_iff] at h
      calc
        _ ≤
            min (emultiplicity (↑p) (q.num * r.den * q.den))
              (emultiplicity (↑p) (↑q.den * r.num * ↑q.den)) :=
          le_min
            (by rw [emultiplicity_mul (a :=_ * _) (Nat.prime_iff_prime_int.1 hp.1), add_comm])
            (by
              rw [mul_assoc,
                  emultiplicity_mul (b := _ * _) (Nat.prime_iff_prime_int.1 hp.1)]
              exact add_le_add_left h _)
        _ ≤ _ := min_le_emultiplicity_add

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
q r : ℚ
hqr : q + r ≠ 0
h : padicValRat p q ≤ padicValRat p r
hq : ¬q = 0
hr : ¬r = 0
hqn : q.num ≠ 0
⊢ padicValRat p q ≤ padicValRat p (q + r)

Tactic:
have hqd : (q.den : ℤ) ≠ 0 := mod_cast Rat.den_nz _

Result:
p : ℕ
hp : Fact (Nat.Prime p)
q r : ℚ
hqr : q + r ≠ 0
h : padicValRat p q ≤ padicValRat p r
hq : ¬q = 0
hr : ¬r = 0
hqn : q.num ≠ 0
hqd : ↑q.den ≠ 0
⊢ padicValRat p q ≤ padicValRat p (q + r)

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
q r : ℚ
hqr : q + r ≠ 0
h : padicValRat p q ≤ padicValRat p r
hq : ¬q = 0
hr : ¬r = 0
hqn : q.num ≠ 0
hqd : ↑q.den ≠ 0
hrn : r.num ≠ 0
hrd : ↑r.den ≠ 0
hqreq : q + r = (q.num * ↑r.den + ↑q.den * r.num) /. (↑q.den * ↑r.den)
hqrd : q.num * ↑r.den + ↑q.den * r.num ≠ 0
⊢ emultiplicity ↑p ↑q.den + emultiplicity (↑p) (q.num * ↑r.den) ≤
    emultiplicity (↑p) (q.num * ↑r.den * ↑q.den + ↑q.den * r.num * ↑q.den)

Tactic:
rw [← q.num_divInt_den, ← r.num_divInt_den, padicValRat_le_padicValRat_iff hqn hrn hqd hrd, ←
  emultiplicity_le_emultiplicity_iff] at h

Result:
p : ℕ
hp : Fact (Nat.Prime p)
q r : ℚ
hqr : q + r ≠ 0
h : emultiplicity (↑p) (q.num * ↑r.den) ≤ emultiplicity (↑p) (r.num * ↑q.den)
hq : ¬q = 0
hr : ¬r = 0
hqn : q.num ≠ 0
hqd : ↑q.den ≠ 0
hrn : r.num ≠ 0
hrd : ↑r.den ≠ 0
hqreq : q + r = (q.num * ↑r.den + ↑q.den * r.num) /. (↑q.den * ↑r.den)
hqrd : q.num * ↑r.den + ↑q.den * r.num ≠ 0
⊢ emultiplicity ↑p ↑q.den + emultiplicity (↑p) (q.num * ↑r.den) ≤
    emultiplicity (↑p) (q.num * ↑r.den * ↑q.den + ↑q.den * r.num * ↑q.den)

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
q r : ℚ
hqr : q + r ≠ 0
h : padicValRat p q ≤ padicValRat p r
hq : q = 0
⊢ padicValRat p q ≤ padicValRat p (q + r)

Tactic:
simpa [hq] using h

Result:
no goals

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
q r : ℚ
hqr : q + r ≠ 0
h : padicValRat p q ≤ padicValRat p r
hq : ¬q = 0
hr : r = 0
⊢ padicValRat p q ≤ padicValRat p (q + r)

Tactic:
simp [hr]

Result:
no goals

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
q r : ℚ
hqr : q + r ≠ 0
h : emultiplicity (↑p) (q.num * ↑r.den) ≤ emultiplicity (↑p) (r.num * ↑q.den)
hq : ¬q = 0
hr : ¬r = 0
hqn : q.num ≠ 0
hqd : ↑q.den ≠ 0
hrn : r.num ≠ 0
hrd : ↑r.den ≠ 0
hqreq : q + r = (q.num * ↑r.den + ↑q.den * r.num) /. (↑q.den * ↑r.den)
hqrd : q.num * ↑r.den + ↑q.den * r.num ≠ 0
⊢ emultiplicity ↑p ↑q.den + emultiplicity (↑p) (q.num * ↑r.den) ≤
    emultiplicity (↑p) (q.num * ↑r.den * ↑q.den + ↑q.den * r.num * ↑q.den)

Tactic:
calc
  _ ≤
      min (emultiplicity (↑p) (q.num * r.den * q.den))
        (emultiplicity (↑p) (↑q.den * r.num * ↑q.den)) :=
    le_min
      (by rw [emultiplicity_mul (a :=_ * _) (Nat.prime_iff_prime_int.1 hp.1), add_comm])
      (by
        rw [mul_assoc,
            emultiplicity_mul (b := _ * _) (Nat.prime_iff_prime_int.1 hp.1)]
        exact add_le_add_left h _)
  _ ≤ _ := min_le_emultiplicity_add

Result:
no goals

Proof state:
p : ℕ
hp : Fact (Nat.Prime p)
q r : ℚ
hqr : q + r ≠ 0
h : padicValRat p q ≤ padicValRat p r
hq : ¬q = 0
hr : ¬r = 0
⊢ padicValRat p q ≤ padicValRat p (q + r)

Tactic:
have hqn : q.num ≠ 0 := Rat.num_ne_zero.2 hq

Result:
p : ℕ
hp : Fact (Nat.Prime p)
q r : ℚ
hqr : q + r ≠ 0
h : padicValRat p q ≤ padicValRat p r
hq : ¬q = 0
hr : ¬r = 0
hqn : q.num ≠ 0
⊢ padicValRat p q ≤ padicValRat p (q + r)