termination_by.lean:9:2-9:18: warning: unused `termination_by`, function is not recursive
termination_by.lean:12:2-12:21: warning: unused `decreasing_by`, function is not recursive
termination_by.lean:15:2-16:21: warning: unused termination hints, function is not recursive
termination_by.lean:19:2-19:18: warning: unused `termination_by`, function is partial
termination_by.lean:22:2-22:21: warning: unused `decreasing_by`, function is partial
termination_by.lean:25:2-26:21: warning: unused termination hints, function is partial
termination_by.lean:29:0-29:16: warning: unused `termination_by`, function is unsafe
termination_by.lean:32:2-32:21: warning: unused `decreasing_by`, function is unsafe
termination_by.lean:35:2-36:21: warning: unused termination hints, function is unsafe
termination_by.lean:40:4-40:20: warning: unused `termination_by`, function is not recursive
termination_by.lean:44:4-44:20: warning: unused `termination_by`, function is not recursive
termination_by.lean:54:2-54:18: warning: unused `termination_by`, function is not recursive
termination_by.lean:62:2-62:23: error: incomplete set of termination hints:
This function is mutually recursive with isOdd, which does not have a termination hint.
The present clause is ignored.
Try this:
  [apply] termination_by x1 => x1