PY
py
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# Compress range
f=lambda a:str(a[0])+"".join(","+str(b) if b!=a+ 1 else ",-"[p+1==a]+str(b) if b+1!=c else "" for (p,c,b,a) in zip(a[0:1]+a,a[2:]+a[-1:],a[1:],a))
print(f([-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20]))
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OUTPUT
Run