Interrupted animations

Any non-discrete animation that is to be abruptly cut off—for the purpose of a repeat or not—will result in an error. This can typically be caused by the use of end,repeatDur and/or syncbases.

<rect width="100" height="100">
    <animate attributeName="x" from="0" to="100" dur="1s" end="0.5s" />
</rect>

Technically, this is possible, but I have no use for this nor the time to calculate the necessary interpolations. Feel free to submit a pull request.