not-sread(positions) input-seq
remove superpositions from the input sequence if they are in 'positions'
index values start from 1, not 0
negative values are also valid, so -1 is last element, -2 is second last element, etc
if out of range, then ignore that index value
'positions' can be a superposition or a sequence, it doesn't matter
if it is a sequence, it is converted to a superposition anyway
the order of the indices in 'positions' is ignored, since they are inserted into a set