So-called "optional rules" are not the only syntactic rules which offer choice. Obligatory as well as optional rules may produce several alternative outputs. In editing, the choice is open to select an output that was not chosen in the original. Such rules are among the copy editor's most common tools. Lexical substitutions are in fact a choice of alternatives, as is the substitution of one tense or aspect by another, and the choice of an alternative article or complementizer.