def decompose_INFO(INFO, position):
    """Keep only data relevant to position.

    >>> decompose_INFO('AN=2;AC=1,1;QD=20.0;AS_QD=2.3,15.7', 2)
    'AC=1;QD=20.0;AS_QD=15.7'
    """
    newinfofields = []
    infofields = INFO.split(";")
    for infofield in infofields:
        annotation, values = infofield.split("=")
        valuelist = values.split(",")
        if len(valuelist) == 2:
            # Split annotations that refer to one allele.
            # example: AC=1,1
            newinfofields.append("=".join([annotation, valuelist[position]]))
        elif annotation == "AN":
            # For now, toss AN until it turns out we need it.
            pass
        elif len(valuelist) == 1:
            # Keep annotations that refer to the entire site.
            # example: QD=30.40
            newinfofields.append(infofield)
    return ";".join(newinfofields)