Never elide semicolons in a projection.
If f is a rank-3 function, the seven possible projections should be written:
f[;;] / not f[]
f[;;c]
f[;b;] / not f[;b]
f[;b;c]
f[a;;] / not f[a]
f[a;;c]
f[a;b;] / not f[a;b]Write projections in full.