Sum.someRight c : Option Nat
evalWithMVar.lean:13:6: error: don't know how to synthesize placeholder
  @Sum.someRight ?m.178 … …
context:
⊢ Type ?
evalWithMVar.lean:13:20: error: don't know how to synthesize placeholder
  @c ?m.178
context:
⊢ Type ?
Sum.someRight c : Option Nat
Sum.someRight c : Option Nat