The C source code for tth and ttm has been taken from the testing line of the Debian Linux distribution which distributes the code under the GPL version 2. The C source has been created using the flex parser generator (by running make after unpacking the sources). The flex sources are not distributed with this R package. A full set of sources including documentation can be obtained from any Debian mirror, e.g. ftp://ftp.debian.org/debian/pool/main/t/tth/ ftp://ftp.at.debian.org/debian/pool/main/t/tth/ Each version which is distributed as part of the R package is also available at http://exams.R-Forge.R-project.org/tth-src/ as original sources, Debian patches and Debian patched version (the latter is used in the R package).