guessLexFailures.lean:11:12-11:46: error: fail to show termination for
  nonTerminating
with errors
argument #1 was not used for structural recursion
  failed to eliminate recursive application
    nonTerminating (Nat.succ n) (Nat.succ m)

argument #2 was not used for structural recursion
  failed to eliminate recursive application
    nonTerminating (Nat.succ n) (Nat.succ m)

structural recursion cannot be used

Could not find a decreasing measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
            x1 x2
1) 11:12-46  ?  ?
Please use `termination_by` to specify a decreasing measure.
guessLexFailures.lean:15:0-18:31: error: Could not find a decreasing measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
            x1 x2
1) 17:12-47  ?  ?
Please use `termination_by` to specify a decreasing measure.
guessLexFailures.lean:20:4-20:15: error: fail to show termination for
  noArguments
with errors
structural recursion cannot be used

well-founded recursion cannot be used, 'noArguments' does not take any (non-fixed) arguments
guessLexFailures.lean:22:4-22:23: error: fail to show termination for
  noNonFixedArguments
with errors
structural recursion cannot be used

well-founded recursion cannot be used, 'noNonFixedArguments' does not take any (non-fixed) arguments
guessLexFailures.lean:27:0-33:31: error: Could not find a decreasing measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
           x1 x2 x3
1) 29:6-25  =  =  =
2) 30:6-23  =  <  _
3) 31:6-23  <  _  _
Please use `termination_by` to specify a decreasing measure.
guessLexFailures.lean:37:0-43:31: error: Could not find a decreasing measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
           x1 x2 x3 x4
1) 39:6-27  =  =  _  =
2) 40:6-25  =  <  _  _
3) 41:6-25  <  _  _  _
Please use `termination_by` to specify a decreasing measure.
guessLexFailures.lean:48:0-54:31: error: Could not find a decreasing measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
           n m l
1) 50:6-25 = = =
2) 51:6-23 = < _
3) 52:6-23 < _ _
Please use `termination_by` to specify a decreasing measure.
guessLexFailures.lean:59:6-59:7: error: fail to show termination for
  Mutual.f
  Mutual.g
  Mutual.h
with errors
structural recursion does not handle mutually recursive functions

Could not find a decreasing measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
Call from Mutual.f to Mutual.f at 61:8-33:
 n m l
 = = =
Call from Mutual.f to Mutual.f at 62:8-31:
 n m l
 = < <
Call from Mutual.f to Mutual.g at 63:8-39:
  n m l
n < ? ?
m ? = ?
H _ _ _
l ? ? <
Call from Mutual.g to Mutual.g at 68:8-35:
 n m H l
 ? ? _ =
Call from Mutual.g to Mutual.g at 69:8-33:
 n m H l
 _ _ _ <
Call from Mutual.g to Mutual.h at 70:8-27:
   n m H l
x1 _ _ _ ?
x2 _ _ _ ?
Call from Mutual.h to Mutual.f at 75:8-33:
  x1 x2
n  _  _
m  _  _
l  _  _
Call from Mutual.h to Mutual.h at 76:8-27:
 x1 x2
  _  _
Call from Mutual.h to Mutual.h at 77:8-27:
 x1 x2
  _  _

Please use `termination_by` to specify a decreasing measure.
guessLexFailures.lean:89:19-89:32: error: fail to show termination for
  DuplicatedCall.f
with errors
argument #1 was not used for structural recursion
  failed to eliminate recursive application
    DuplicatedCall.f (n + 2) (m + 1)

argument #2 was not used for structural recursion
  failed to eliminate recursive application
    DuplicatedCall.f (n + 2) (m + 1)

structural recursion cannot be used

Could not find a decreasing measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
            x1 x2
1) 89:19-32  ?  ?
Please use `termination_by` to specify a decreasing measure.
guessLexFailures.lean:100:0-103:31: error: Could not find a decreasing measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
              n m
1) 101:83-105 ? ?
Please use `termination_by` to specify a decreasing measure.
guessLexFailures.lean:113:14-113:31: error: failed to prove termination, possible solutions:
  - Use `have`-expressions to prove the remaining goals
  - Use `termination_by` to specify a different well-founded relation
  - Use `decreasing_by` to specify your own tactic for discharging this kind of goal
n✝ m n : Nat
⊢ n < n✝
guessLexFailures.lean:119:0-123:31: error: Could not find a decreasing measure.
The arguments relate at each recursive call as follows:
(<, ≤, =: relation proved, ? all proofs failed, _: no proof attempted)
             n m
1) 121:31-54 ? ?
Please use `termination_by` to specify a decreasing measure.
guessLexFailures.lean:131:14-131:31: error: failed to prove termination, possible solutions:
  - Use `have`-expressions to prove the remaining goals
  - Use `termination_by` to specify a different well-founded relation
  - Use `decreasing_by` to specify your own tactic for discharging this kind of goal
m n✝ n : Nat
⊢ n < n✝ + 1