Negative spacing

Created by: fred-wang

"The present document assumes that W,w,a,b,A,BW,w,a,b,A,B, italic correction and top accent attachment are nonnegative values. If computations result in negative values, then they must be interpreted as being zero. Note that some MathML elements such as mspace or mpadded may set box dimensions to negative values."

In the future, we should explain how to handle negative spacing.