PrependTo[s, elem] prepends elem to the value of s, and resets s to the result.
PrependTo[s, elem] is equivalent to s = Prepend[s, elem].
PrependTo[s, elem] does not evaluate s.
You can use PrependTo repeatedly to build up a list, though Sow and Reap will usually be more efficient.
PrependTo works on SparseArray objects, returning ordinary lists if necessary.
See Section 2.5.4.
See also: AppendTo, Sow.
New in Version 1.