https://issues.dlang.org/show_bug.cgi?id=17238

          Issue ID: 17238
           Summary: r.lookaround!(a, b) returns a range derived from r
                    that allows a elements of lookahead and b elements of
                    lookback
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody@puremagic.com
          Reporter: andrei@erdani.com

Inspired by https://github.com/dlang/phobos/pull/5153, a great range type would be one that offers fixed lookaround. It works like any other range but has supplemental functions for lookahead and lookback.

r.lookahead!(a, b) yields a range looking up to a elements ahead and up to b
elements back. These are accessible through two new APIs, e.g. ahead(k) and
behind(k).

If r is a random-access range, the resulting range does NOT copy any elements and simply provides APIs on top of the random access primitives.

If r is weaker than random-access, it embeds a fixed-size buffer and moves through it with minimal effort.

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