Try these:
  [apply] [grind
    =] for pattern: [@HAppend.hAppend (List #3) (List _) (List _) _ (@HAppend.hAppend (List _) (List _) (List _) _ #2 #1) #0]
  [apply] [grind
    =_] for pattern: [@HAppend.hAppend (List #3) (List _) (List _) _ #2 (@HAppend.hAppend (List _) (List _) (List _) _ #1 #0)]
Try these:
  [apply] [grind =] for pattern: [@HAppend.hAppend (List #3) (List _) (List _) _ (@List.cons _ #2 #1) #0]
  [apply] [grind =_] for pattern: [@List.cons #3 #2 (@HAppend.hAppend (List _) (List _) (List _) _ #1 #0)]
Try this:
  [apply] [grind =] for pattern: [@HAppend.hAppend (List #1) (List _) (List _) _ (@List.nil _) #0]