nested_match.lean:7:13: 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:
f : ℕ → ℕ,
n m : ℕ
⊢ m < n
nested_match.lean:25:7: error: failed to prove recursive application is decreasing, well founded relation
  @has_well_founded.r (psum ℕ ℕ)
    (@has_well_founded_of_has_sizeof (psum ℕ ℕ) (@psum.has_sizeof ℕ ℕ nat.has_sizeof 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:
n : ℕ
⊢ 1 < 1