RFC: `if let` chains

Decrease redundancy in cases like

if let c a := x then
  if q a then
    y
  else z
else z
...

by supporting mixed let/condition chains like

if let c a := x && q a then
  y
else z
...

This is a syntactic idea I thought was too wild to even propose, until I learned that Rust will have it, so obviously it must be a good idea.
Probably even more than for Rust though, parsing this is a bit of an issue. The cleanest approach I could come up with so far is to keep basic if syntax intact and introduce new chain syntax for subterms of precedence greater than &&/∧ only.

syntax letChainElem := "let " term " := " term:36  -- "&&"/"∧" have precedence 35
syntax (priority := low) "if " sepBy1(letChainElem <|> term:36, "&&", " && " <|> " ∧ ") " then " term " else " term : term
#check if let x := a && p x then c else d  -- parsed as elements `[let x := a, p x]` (after removing current `if let` syntax)
#check if let x := a && let y := b then c else d  -- parsed as elements `[let x := a, let y := b]`
#check if a || b then c else d  -- parsed by basic syntax
#check if a || b && let x := a then c else d  -- parse error, incomplete term `let`
#check if (a || b) && let x := a then c else d  -- parsed as elements `[a || b, let x := a]`
#check if a then b else c  -- parsed by both, disambiguated by priority

Desugaring: obvious, modulo introducing join points

No implementation files, entry points, or tests are named. Start by reviewing the proposed syntax, precedence examples, and desugaring discussion in this issue; done would require an agreed parsing and implementation design plus validation, but the payload does not specify where those live.