Sum.someRight c : Option Nat
evalWithMVar.lean:13:6-13:21: error: don't know how to synthesize implicit argument
  @Sum.someRight ?m Nat c
context:
⊢ Type u_1
evalWithMVar.lean:13:20-13:21: error: don't know how to synthesize implicit argument
  @c ?m
context:
⊢ Type u_1
Sum.someRight c : Option Nat
Sum.someRight c : Option Nat