wf1.lean:1:4-1:5: error: fail to show termination for
  g
with errors
argument #1 was not used for structural recursion
  failed to eliminate recursive application
    g (x - 1)

argument #2 was not used for structural recursion
  insufficient number of parameters at recursive application 
    g (x - 1)

structural recursion cannot be used

failed to prove termination, use `termination_by` to specify a well-founded relation