Bad use of Seq.item in sumSeq example code

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sumSeq uses Seq.item to get to the item of interest in sequence -- this means it has to calculate all the intermediate items afresh every time. This makes it very slow and a bad example for someone just learning seqs/IEnumerables. A better implementation of sumSeq would be something like:

    let sumSeq length sequence =
        sequence
        |> Seq.skip 1                      // sumSeq uses Seq.item (index+1) -- so it always skips first item
        |> Seq.truncate length      // don't take more than length items
        |> Seq.scan (+) 0.0            // generate running sums
        |> Seq.skip 1                     // skip the initial 0.0 from sequence of running sums

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• ID: 67fae525-473a-3072-b024-6d91d0e033b9
• Version Independent ID: 6e162795-52bd-f9df-4105-6919c1c72c88
• Content: Sequences - F#
• Content Source: docs/fsharp/language-reference/sequences.md
• Product: dotnet-fsharp
• GitHub Login: @cartermp
• Microsoft Alias: dotnetcontent

[BillWagner]

Thanks @glchapman

This is a great suggestion. I've added it to our backlog, and added the "help wanted" label if anyone wants to submit a PR in the interim.