bitmap_fold — fold larger bitmap into smaller, modulo specified size
| void bitmap_fold ( | unsigned long * dst, | 
| const unsigned long * orig, | |
| unsigned int sz, | |
| unsigned int nbits ); | 
dstresulting smaller bitmap
origoriginal larger bitmap
szspecified size
nbitsnumber of bits in each of these bitmaps
   For each bit oldbit in orig, set bit oldbit mod sz in dst.
   Clear all other bits in dst.  See further the comment and
   Example [2] for bitmap_onto for why and how to use this.