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