>What is the feature?
It is the use of routine names after strings in amongs. To quote from the
manual:
In its most general form, each string  'Sij'  may be optionally followed by
a routine name, 
    among( (C)
           'S11' R11 'S12' R12 ... (C1)
           'S21' R21 'S22' R22 ... (C2)
           ...
           'Sn1' Rn1 'Sn2' Rn1 ... (Cn)
         )
So here each  Rij  is either a routine name or is null. If null, it is
equivalent to a routine which simply returns signal t.
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