Main sequence mechanism was unidirectional. This meant that complex programs had to be physically lengthy. A program loop was accomplished by loop unrolling or by joining the end of the paper tape containing the program back to the beginning of the tape (literally creating a loop). At first, conditional branching in the Mark I was performed manually. Later modifications in 1946 introduced automatic program branching (by subroutine call). The first programmers of the Mark I were computing pioneers Richard Milton Bloch, Robert Campbell, and Grace Hopper. There was also a small technical team whose purpose was to actually operate the machine, some of whom were IBM employees before being required to join the Navy to work on the machine. This technical team was not informed of the purpose of their work while at Harvard.