test(tests/lean/run): add another example on how to deal with ambiguity

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

tests/lean/run/e12.lean

@@ -0,0 +1,55 @@
+precedence `+`:65
+
+namespace nat
+  variable nat : Type.{1}
+  variable add : nat → nat → nat
+  infixl + := add
+end
+
+namespace int
+  using nat (nat)
+  variable int : Type.{1}
+  variable add : int → int → int
+  infixl + := add
+  variable of_nat : nat → int
+  coercion of_nat
+end
+
+variables n m : nat.nat
+variables i j : int.int
+
+section
+  using [notation] nat
+  using [notation] int
+  using [decls] nat
+  using [decls] int
+  check n+m
+  check i+j
+  --  check i+n -- Error
+end
+
+namespace int
+  using [decls] nat (nat)
+  -- Here is a possible trick for this kind of configuration
+  definition add_ni (a : nat) (b : int) := (of_nat a) + b
+  definition add_in (a : int) (b : nat) := a + (of_nat b)
+  infixl + := add_ni
+  infixl + := add_in
+end
+
+section
+  using [notation] nat
+  using [notation] int
+  using [declarations] nat
+  using [declarations] int
+  check n+m
+  check i+n
+  check n+i
+end
+
+section
+  using nat
+  using int
+  check n+m
+  check i+n
+end