alignment = {
qname : string option;
flags : Flags.t;
rname : string option;
pos : int option;
mapq : int option;
cigar : cigar_op list;
rnext : rnext option;
pnext : int option;
tlen : int option;
seq: string option;
qual: Biocaml_phred_score.t list;
optional_fields : optional_field list;
}