Agreed. When the manual says you can pattern match with term rewriting macros, I think of Erlang pattern matching. Would something like this be possible?

    template mysum{a = @[]}(a: seq[int]): int = 0
    template mysum{a}(a: seq[int]): int =
      while a.low <= a.high:
        a[a.low] + mysum(a[a.low + 1 .. a.high])

    var x: seq[int] = @[1, 2, 3]

    echo mysum(x)