code: function pos2index(pos, prelen, linelen) {
    return Number(prelen) + Number(pos) -1 + Math.floor( (pos -1)/linelen );
  }
docs: /* static functions 
  FASTAReader.pos2index
  convert DNA base position to character index
  @param number  pos     : DNA base position
  @param number  prelen  : header data length
  @param number  linelen : one line length
  @return number : character index