abbrev_paren.hlean:5:54: error: don't know how to synthesize placeholder
C : Precategory
⊢ C = Precategory.mk (carrier C) C
abbrev_paren.hlean:5:54: error: failed to add declaration 'example' to environment, value has metavariables
remark: set 'formatter.hide_full_terms' to false to see the complete term
  ?M_1