relSidesIfSymm? — Mathlib

Lean4
/-- If `e` is the form `@R .. x y`, where `R` is a symmetric
relation, return `some (R, x, y)`.
As a special case, if `e` is `@HEq α a β b`, return ``some (`HEq, a, b)``. -/
def _root_.Lean.Expr.relSidesIfSymm? (e : Expr) : MetaM (Option (Name × Expr × Expr)) := do
  if let some (_, lhs, rhs) := e.eq? then
    return (``Eq, lhs, rhs)
  if let some (lhs, rhs) := e.iff? then
    return (``Iff, lhs, rhs)
  if let some (_, lhs, _, rhs) := e.heq? then
    return (``HEq, lhs, rhs)
  if let .app (.app rel lhs) rhs := e then
    unless (← (symmExt.getState (← getEnv)).getMatch rel).isEmpty do
      match rel.getAppFn.constName? with
      | some n =>
        return some (n, lhs, rhs)
      | none =>
        return none
  return none
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