subset_error.lean:18:51: error: type mismatch at application
  x ∈ S
term
  S
has type
  set A
but is expected to have type
  finset A
subset_error.lean:18:51: previous error additional information
none of the overloads is applicable: finset.mem set.mem
subset_error.lean:18:66: previous error additional information
none of the overloads is applicable: finset.subset set.subset