This directory contains files from PEGTL 2.4.0:
 LICENSE and include/tao/* except include/tao/pegtl/contrib
with minor local modifications:
- changed formatting of error messages,
- using std::system_error instead of pegtl::input_error.

PEGTL can be updated to v3 only when gemmi drops support for C++11/14.
https://github.com/project-gemmi/gemmi/issues/212
