Mergesort
    
LEGEND:
Last column:
 (   call to mergesort
 )   return from mergesort via base case (sub-array of len 1)
 )+  return from mergesort, after copying back the recombined halves
pre  before the two recursive "conquer" calls
 L   after having sorted the left half
 R   after having sorted the right half
 C   before copyback, after recombination in temp area

Within array:
 [   beginning of portion to be sorted
 ]   end of portion to be sorted
 |   split point between left and right halves
 :   separator between data portion and temp storage

[ 5   2   4   6   1   3 ]                            ( 
[ 5   2   4 | 6   1   3 ]                           pre
[ 5   2   4 ] 6   1   3 :                            ( 
[ 5 | 2   4 ] 6   1   3 :                           pre
[ 5 ] 2   4   6   1   3 :                            ( 
[ 5 ] 2   4   6   1   3 :                            ) 
[ 5 | 2   4 ] 6   1   3 :                            L 
  5 [ 2   4 ] 6   1   3 :                            ( 
  5 [ 2 | 4 ] 6   1   3 :                           pre
  5 [ 2 ] 4   6   1   3 :                            ( 
  5 [ 2 ] 4   6   1   3 :                            ) 
  5 [ 2 | 4 ] 6   1   3 :                            L 
  5   2 [ 4 ] 6   1   3 :                            ( 
  5   2 [ 4 ] 6   1   3 :                            ) 
  5 [ 2 | 4 ] 6   1   3 :                            R 
  5 [ 2   4 ] 6   1   3 : 2   4                      C 
  5 [ 2   4 ] 6   1   3 : 2   4                      )+
[ 5 | 2   4 ] 6   1   3 : 2   4                      R 
[ 5   2   4 ] 6   1   3 : 2   4   5                  C 
[ 2   4   5 ] 6   1   3 : 2   4   5                  )+
[ 2   4   5 | 6   1   3 ] 2   4   5                  L 
  2   4   5 [ 6   1   3 ] 2   4   5                  ( 
  2   4   5 [ 6 | 1   3 ] 2   4   5                 pre
  2   4   5 [ 6 ] 1   3 : 2   4   5                  ( 
  2   4   5 [ 6 ] 1   3 : 2   4   5                  ) 
  2   4   5 [ 6 | 1   3 ] 2   4   5                  L 
  2   4   5   6 [ 1   3 ] 2   4   5                  ( 
  2   4   5   6 [ 1 | 3 ] 2   4   5                 pre
  2   4   5   6 [ 1 ] 3 : 2   4   5                  ( 
  2   4   5   6 [ 1 ] 3 : 2   4   5                  ) 
  2   4   5   6 [ 1 | 3 ] 2   4   5                  L 
  2   4   5   6   1 [ 3 ] 2   4   5                  ( 
  2   4   5   6   1 [ 3 ] 2   4   5                  ) 
  2   4   5   6 [ 1 | 3 ] 2   4   5                  R 
  2   4   5   6 [ 1   3 ] 1   3   5                  C 
  2   4   5   6 [ 1   3 ] 1   3   5                  )+
  2   4   5 [ 6 | 1   3 ] 1   3   5                  R 
  2   4   5 [ 6   1   3 ] 1   3   6                  C 
  2   4   5 [ 1   3   6 ] 1   3   6                  )+
[ 2   4   5 | 1   3   6 ] 1   3   6                  R 
[ 2   4   5   1   3   6 ] 1   2   3   4   5   6      C 
[ 1   2   3   4   5   6 ] 1   2   3   4   5   6      )+