Currently, we forbid negative and relative values for mo@lspace/mo@rspace. Implementing negative values may require a non-trivial work on MathML boxes, perhaps related to bug 717546. Allowing relative values may not give the right result when ProcessOperatorData is called several times (see bug 677036 comment 17). See also this thread: http://lists.w3.org/Archives/Public/www-math/2012Oct/0014.html
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