code: def pattern_error(self, original, loc, value_var, check_var):
          
          base_line = clean(self.reformat(getline(loc, original)))
          line_wrap = self.wrap_str_of(base_line)
          repr_wrap = self.wrap_str_of(ascii(base_line))
          return (
              "if not " + check_var + ":\n" + openindent
              + match_err_var + ' = _coconut_MatchError("pattern-matching failed for " '
              + repr_wrap + ' " in " + _coconut.repr(_coconut.repr(' + value_var + ")))\n"
              + match_err_var + ".pattern = " + line_wrap + "\n"
              + match_err_var + ".value = " + value_var
              + "\nraise " + match_err_var + "\n" + closeindent
          )
docs: Construct a pattern-matching error message.