+probability. Because this file contains all alignment lines produced
+by bowtie or user-specified aligners, it can also be used as a
+replacement of the aligner generated BAM/SAM file. For paired-end
+reads, if one mate has alignments but the other does not, this file
+marks the alignable mate as "unmappable" (flag bit 0x4) and appends an
+optional field "Z0:A:!".