list.map : (?M_1 → ?M_2) → list ?M_1 → list ?M_2
field_access.lean:5:8: error: invalid projection, structure expected
  l
has type
  list ℕ
field_access.lean:7:13: error: invalid projection, structure has only 2 field(s)
  (1, 2)
which has type
  ?m_1 × ?m_2
field_access.lean:10:1: error: invalid '^.' notation, 'forr' is not a valid "field" because environment does not contain 'list.forr'
  l
which has type
  list ℕ
field_access.lean:13:1: error: invalid '^.' notation, type is not of the form (C ...) where C is a constant
  a
has type
  A