Designer, implementor, user, writer

Fragment by Donald Knuth

Thus, I came to the conclusion that the designer of a new system must not only be the implementor and the first large-scale user; the designer should also write the first user manual. The separation of any of these four components would have hurt TeX significantly. If I had not participated fully in all these activities, literally hundreds of improvements would never have been made, because I would never have thought of them or perceived why they were important.

  1. ​Eating your own dog food​

From Software—Practice and Experience, Vol. 19: The Errors of TeX.