\ifundefined\ifundefined{cs} t \else f \fi expands the
t text if the control sequence \cs is undefined or
has been \let to \relax, and the f text otherwise.
Since \ifundefined is not a primitive conditional, it cannot be
used in places where TeX might skip tokens “at high speed”, e.g.,
within another conditional—TeX can't match up the \if's and
\fi's.
This macro was taken directly from The TeXbook, page 308.