440.lean:1:21-1:22: error: don't know how to synthesize placeholder
context:
⊢ True
440.lean:4:29-4:30: error: don't know how to synthesize placeholder
context:
x : Nat
⊢ Nat
f (x : Nat) : Nat
440.lean:11:0-11:5: error: aborting evaluation since the expression depends on the 'sorry' axiom, which can lead to runtime instability and crashes.

To attempt to evaluate anyway despite the risks, use the '#eval!' command.