File "hello.ml", line 18, characters 20-27:
Error: These are the contents of the main error message. It is very long and
       should wrap across several lines.
File "hello.ml", line 20, characters 4-8:
  A located first sub-message.
File "hello.ml", line 20, characters 6-8:
  Longer sub-messages that do not fit on the same line as the location get
  indented.
This second sub-message does not have a location;
ghost locations of submessages are not printed.