1917.lean:4:21: error: failed to prove recursive application is decreasing, well founded relation
  @has_well_founded.r ℕ (@has_well_founded_of_has_sizeof ℕ nat.has_sizeof)
Possible solutions: 
  - Use 'using_well_founded' keyword in the end of your definition to specify tactics for synthesizing well founded relations and decreasing proofs.
  - The default decreasing tactic uses the 'assumption' tactic, thus hints (aka local proofs) can be provided using 'have'-expressions.
The nested exception contains the failure state for the decreasing tactic.
nested exception message:
failed
state:
foo : ℕ → false,
x y : ℕ
⊢ y < x