def index_which(ol, value, which):
    """
        from elist.elist import *
        ol = [1,'a',3,'a',4,'a',5]
        index_which(ol,'a',0)
        index_which(ol,'a',1)
        index_which(ol,'a',2)
        index_which(ol,'a',3) == None
    """
    length = ol.__len__()
    seq = -1
    for i in range(0, length):
        if value == ol[i]:
            seq = seq + 1
            if seq == which:
                return i
            else:
                pass
        else:
            pass
    return None
