notation let' (x,y) := v; b

which expands to
```
let p := v
let x:= p.1
let x:= p.2
b
```
rather then to match like normal let binding

SciLean/Meta/Notation/Let'.lean

@@ -0,0 +1,52 @@
+
+namespace SciLean
+
+/-- Variant of let binding for product pattern that does not desugar into match but into
+bunch of other let bindings and projections.
+
+For examples
+```
+let' (x,y) := v;
+b
+```
+expands into
+```
+let p := v
+let x := p.1
+let y := p.2
+b
+```
+-/
+syntax (name:=let'_syntax) "let'" "(" ident,* ")" ":=" term ";" term : term
+
+-- TODO: make sure it works for arbitrary number of identitfiers and ideally for arbitrary pattern.
+@[inherit_doc let'_syntax]
+macro_rules
+| `(let' ($x,$y) := $v; $b) =>
+  `(let x := $v
+    let $x := Prod.fst x
+    let $y := Prod.snd x
+    $b)
+| `(let' ($x,$y,$z) := $v; $b) =>
+  `(let x := $v
+    let $x := Prod.fst x
+    let $y := Prod.fst (Prod.snd x)
+    let $z := Prod.snd (Prod.snd x)
+    $b)
+
+| `(let' ($x,$y,$z,$w) := $v; $b) =>
+  `(let x := $v
+    let $x := Prod.fst x
+    let $y := Prod.fst (Prod.snd x)
+    let $z := Prod.fst (Prod.snd (Prod.snd x))
+    let $w := Prod.snd (Prod.snd (Prod.snd x))
+    $b)
+
+
+@[inherit_doc let'_syntax]
+macro "let'" "(" "(" x:ident "," y:ident ")" "," z:ident ")" ":=" v:term ";" b:term : term =>
+  `(let x := $v
+    let $x := x.1.1
+    let $y := x.1.2
+    let $z := x.2
+    $b)