def pattern_error(self, original, loc, value_var, check_var):
        """Construct a pattern-matching error message."""
        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
        )
Construct a pattern-matching error message.