def Lean.mkRecAppWithSyntax (e : Lean.Expr) (stx : Lean.Syntax) : Lean.Expr

We store the syntax at recursive applications to be able to generate better error messages when performing well-founded and structural recursion.

def Lean.getRecAppSyntax? (e : Lean.Expr) : Option Lean.Syntax

Retrieve (if available) the syntax object attached to a recursive application.