REDUCE includes a module for printing REDUCE syntax in a standard format. This module is activated by the switch PRET, which is normally off.
Since the system converts algebraic input into an equivalent symbolic form, the printing program tries to interpret this as an algebraic expression before printing it. In most cases, this can be done successfully. However, there will be occasional instances where results are printed in symbolic mode form that bears little resemblance to the original input, even though it is formally equivalent.
If you want to prettyprint a whole file, say off output,msg;MSG and (hopefully) only clean output will result. Unlike DEFN, input is also evaluated with PRET on.