+# error handling methods:
+#
+# Error in input file, while $err_file and $. set, eg in most of
+# process_input_mk:
+# err "message";
+#
+# Other input or usage errors:
+# die "subdirmk: $file:$lno: problem\n";
+# die "subdirmk: some problem not locatable in that way\n";
+#
+# Usage error:
+# die "subdirmk $0: explanation of problem\n";
+#
+# System call error (not ENOENT) accessing input/output files:
+# die "description of problem eg maybe erbing noun: $!\n";
+#
+# Bug detedcted in `generate':
+# die "internal error (some information)?"; # or similar
+