def spacerGC(L, spacerOffset=6, spacerLength=3):
    """
    spacerGC takes as input a list of [15 bp GBSs (strings)] and
    returns the number of sequences that have 0,1,2,3 G/Cs in the 3 bp spacer 
    as an array in that order
    """
    # gc counts of 0, 1, 2, 3
    GCcounts = [0, 0, 0, 0]
    for s in L:
        spacer = s[0][spacerOffset:spacerOffset+spacerLength].upper()
        GCs = spacer.count('C') + spacer.count('G')
        GCcounts[GCs] += 1
    return GCcounts