-- Mathlib/Data/Real/EReal.lean
theorem induction₂ {P : EReal → EReal → Prop} (top_top : P ⊤ ⊤) (top_pos : ∀ x : ℝ, 0 < x → P ⊤ x)
    (top_zero : P ⊤ 0) (top_neg : ∀ x : ℝ, x < 0 → P ⊤ x) (top_bot : P ⊤ ⊥)
    (pos_top : ∀ x : ℝ, 0 < x → P x ⊤) (pos_bot : ∀ x : ℝ, 0 < x → P x ⊥) (zero_top : P 0 ⊤)
    (coe_coe : ∀ x y : ℝ, P x y) (zero_bot : P 0 ⊥) (neg_top : ∀ x : ℝ, x < 0 → P x ⊤)
    (neg_bot : ∀ x : ℝ, x < 0 → P x ⊥) (bot_top : P ⊥ ⊤) (bot_pos : ∀ x : ℝ, 0 < x → P ⊥ x)
    (bot_zero : P ⊥ 0) (bot_neg : ∀ x : ℝ, x < 0 → P ⊥ x) (bot_bot : P ⊥ ⊥) : ∀ x y, P x y
  | ⊥, ⊥ => bot_bot
  | ⊥, (y : ℝ) => by
    rcases lt_trichotomy y 0 with (hy | rfl | hy)
    exacts [bot_neg y hy, bot_zero, bot_pos y hy]
  | ⊥, ⊤ => bot_top
  | (x : ℝ), ⊥ => by
    rcases lt_trichotomy x 0 with (hx | rfl | hx)
    exacts [neg_bot x hx, zero_bot, pos_bot x hx]
  | (x : ℝ), (y : ℝ) => coe_coe _ _
  | (x : ℝ), ⊤ => by
    rcases lt_trichotomy x 0 with (hx | rfl | hx)
    exacts [neg_top x hx, zero_top, pos_top x hx]
  | ⊤, ⊥ => top_bot
  | ⊤, (y : ℝ) => by
    rcases lt_trichotomy y 0 with (hy | rfl | hy)
    exacts [top_neg y hy, top_zero, top_pos y hy]
  | ⊤, ⊤ => top_top

Proof state:
P : EReal → EReal → Prop
top_top : P ⊤ ⊤
top_pos : ∀ (x : ℝ), 0 < x → P ⊤ ↑x
top_zero : P ⊤ 0
top_neg : ∀ x < 0, P ⊤ ↑x
top_bot : P ⊤ ⊥
pos_top : ∀ (x : ℝ), 0 < x → P ↑x ⊤
pos_bot : ∀ (x : ℝ), 0 < x → P ↑x ⊥
zero_top : P 0 ⊤
coe_coe : ∀ (x y : ℝ), P ↑x ↑y
zero_bot : P 0 ⊥
neg_top : ∀ x < 0, P ↑x ⊤
neg_bot : ∀ x < 0, P ↑x ⊥
bot_top : P ⊥ ⊤
bot_pos : ∀ (x : ℝ), 0 < x → P ⊥ ↑x
bot_zero : P ⊥ 0
bot_neg : ∀ x < 0, P ⊥ ↑x
bot_bot : P ⊥ ⊥
x : ℝ
⊢ P ↑x ⊤

Tactic:
rcases lt_trichotomy x 0 with (hx | rfl | hx)

Result:
case inl
P : EReal → EReal → Prop
top_top : P ⊤ ⊤
top_pos : ∀ (x : ℝ), 0 < x → P ⊤ ↑x
top_zero : P ⊤ 0
top_neg : ∀ x < 0, P ⊤ ↑x
top_bot : P ⊤ ⊥
pos_top : ∀ (x : ℝ), 0 < x → P ↑x ⊤
pos_bot : ∀ (x : ℝ), 0 < x → P ↑x ⊥
zero_top : P 0 ⊤
coe_coe : ∀ (x y : ℝ), P ↑x ↑y
zero_bot : P 0 ⊥
neg_top : ∀ x < 0, P ↑x ⊤
neg_bot : ∀ x < 0, P ↑x ⊥
bot_top : P ⊥ ⊤
bot_pos : ∀ (x : ℝ), 0 < x → P ⊥ ↑x
bot_zero : P ⊥ 0
bot_neg : ∀ x < 0, P ⊥ ↑x
bot_bot : P ⊥ ⊥
x : ℝ
hx : x < 0
⊢ P ↑x ⊤

case inr.inl
P : EReal → EReal → Prop
top_top : P ⊤ ⊤
top_pos : ∀ (x : ℝ), 0 < x → P ⊤ ↑x
top_zero : P ⊤ 0
top_neg : ∀ x < 0, P ⊤ ↑x
top_bot : P ⊤ ⊥
pos_top : ∀ (x : ℝ), 0 < x → P ↑x ⊤
pos_bot : ∀ (x : ℝ), 0 < x → P ↑x ⊥
zero_top : P 0 ⊤
coe_coe : ∀ (x y : ℝ), P ↑x ↑y
zero_bot : P 0 ⊥
neg_top : ∀ x < 0, P ↑x ⊤
neg_bot : ∀ x < 0, P ↑x ⊥
bot_top : P ⊥ ⊤
bot_pos : ∀ (x : ℝ), 0 < x → P ⊥ ↑x
bot_zero : P ⊥ 0
bot_neg : ∀ x < 0, P ⊥ ↑x
bot_bot : P ⊥ ⊥
⊢ P ↑0 ⊤

case inr.inr
P : EReal → EReal → Prop
top_top : P ⊤ ⊤
top_pos : ∀ (x : ℝ), 0 < x → P ⊤ ↑x
top_zero : P ⊤ 0
top_neg : ∀ x < 0, P ⊤ ↑x
top_bot : P ⊤ ⊥
pos_top : ∀ (x : ℝ), 0 < x → P ↑x ⊤
pos_bot : ∀ (x : ℝ), 0 < x → P ↑x ⊥
zero_top : P 0 ⊤
coe_coe : ∀ (x y : ℝ), P ↑x ↑y
zero_bot : P 0 ⊥
neg_top : ∀ x < 0, P ↑x ⊤
neg_bot : ∀ x < 0, P ↑x ⊥
bot_top : P ⊥ ⊤
bot_pos : ∀ (x : ℝ), 0 < x → P ⊥ ↑x
bot_zero : P ⊥ 0
bot_neg : ∀ x < 0, P ⊥ ↑x
bot_bot : P ⊥ ⊥
x : ℝ
hx : 0 < x
⊢ P ↑x ⊤

Proof state:
case inl
P : EReal → EReal → Prop
top_top : P ⊤ ⊤
top_pos : ∀ (x : ℝ), 0 < x → P ⊤ ↑x
top_zero : P ⊤ 0
top_neg : ∀ x < 0, P ⊤ ↑x
top_bot : P ⊤ ⊥
pos_top : ∀ (x : ℝ), 0 < x → P ↑x ⊤
pos_bot : ∀ (x : ℝ), 0 < x → P ↑x ⊥
zero_top : P 0 ⊤
coe_coe : ∀ (x y : ℝ), P ↑x ↑y
zero_bot : P 0 ⊥
neg_top : ∀ x < 0, P ↑x ⊤
neg_bot : ∀ x < 0, P ↑x ⊥
bot_top : P ⊥ ⊤
bot_pos : ∀ (x : ℝ), 0 < x → P ⊥ ↑x
bot_zero : P ⊥ 0
bot_neg : ∀ x < 0, P ⊥ ↑x
bot_bot : P ⊥ ⊥
x : ℝ
hx : x < 0
⊢ P ↑x ⊤

case inr.inl
P : EReal → EReal → Prop
top_top : P ⊤ ⊤
top_pos : ∀ (x : ℝ), 0 < x → P ⊤ ↑x
top_zero : P ⊤ 0
top_neg : ∀ x < 0, P ⊤ ↑x
top_bot : P ⊤ ⊥
pos_top : ∀ (x : ℝ), 0 < x → P ↑x ⊤
pos_bot : ∀ (x : ℝ), 0 < x → P ↑x ⊥
zero_top : P 0 ⊤
coe_coe : ∀ (x y : ℝ), P ↑x ↑y
zero_bot : P 0 ⊥
neg_top : ∀ x < 0, P ↑x ⊤
neg_bot : ∀ x < 0, P ↑x ⊥
bot_top : P ⊥ ⊤
bot_pos : ∀ (x : ℝ), 0 < x → P ⊥ ↑x
bot_zero : P ⊥ 0
bot_neg : ∀ x < 0, P ⊥ ↑x
bot_bot : P ⊥ ⊥
⊢ P ↑0 ⊤

case inr.inr
P : EReal → EReal → Prop
top_top : P ⊤ ⊤
top_pos : ∀ (x : ℝ), 0 < x → P ⊤ ↑x
top_zero : P ⊤ 0
top_neg : ∀ x < 0, P ⊤ ↑x
top_bot : P ⊤ ⊥
pos_top : ∀ (x : ℝ), 0 < x → P ↑x ⊤
pos_bot : ∀ (x : ℝ), 0 < x → P ↑x ⊥
zero_top : P 0 ⊤
coe_coe : ∀ (x y : ℝ), P ↑x ↑y
zero_bot : P 0 ⊥
neg_top : ∀ x < 0, P ↑x ⊤
neg_bot : ∀ x < 0, P ↑x ⊥
bot_top : P ⊥ ⊤
bot_pos : ∀ (x : ℝ), 0 < x → P ⊥ ↑x
bot_zero : P ⊥ 0
bot_neg : ∀ x < 0, P ⊥ ↑x
bot_bot : P ⊥ ⊥
x : ℝ
hx : 0 < x
⊢ P ↑x ⊤

Tactic:
exacts [neg_top x hx, zero_top, pos_top x hx]

Result:
no goals