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sread(positions) input-seq
replace positions with their index in the input sequence
we try to preserve the 'structure' of '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, or not a number, return |> for that slot

-- preserve superposition structure of 'positions':
sread(|1> + |3> + |5>) (|one> . |two> . |three> . |four> . |five> . |six> . |seven>)
    |one> + |three> + |five>

-- preserve sequence structure of 'positions':
sread(|1> . |3> . |5>) (|one> . |two> . |three> . |four> . |five> . |six> . |seven>)
    |one> . |three> . |five>

-- handle negative indices:
sread(|-1> . |-2> . |-3>) (|one> . |two> . |three> . |four> . |five> . |six> . |seven>)
    |seven> . |six> . |five>

-- if out of range, or not a number, return |> for that slot:
sread(|1> . |fish> . |11> . |4>) (|one> . |two> . |three> . |four> . |five> . |six> . |seven>)
    |one> . |> . |> . |four>