\code{\var{x} = \var{i} + \var{n}*\var{k}} such that \code{0}
\code{<=} \var{n} \code{<} \code{abs(i-j)}. If \var{i} or \var{j}
is greater than \code{len(\var{s})}, use \code{len(\var{s})}. If
- \var{i} or \var{j} are ommitted then they become ``end'' values
- (which end depends on the sign of \var{k}).
+ \var{i} or \var{j} are omitted then they become ``end'' values
+ (which end depends on the sign of \var{k}). Note, \var{k} cannot
+ be zero.
\end{description}