importance of tie bars?

Hi all.

I've been talking to some people using MathML for chemistry lately, and got a question about tie bars, which can't usually be represented in MathML.  If I understand correctly, tie bars look like a horizontal bracket (i.e. '[') under an equation, and seem to be primarily used to point to two terms in an equation that are related in some way.  I'm not sure of this, but there is also probably a requirement to label the horizontal part of the notation.  E.g.

(x_0 + ... + x_i + ... x_n)^a = (x_0 + ... + x_i + ... x_n)^b
              |                               |
               -------------------------------
                       ith summand

I hope this comes out okay as ASCII art.

However, by their nature, they do not respect the equation nesting structure, and so are tricky in MathML.  Something like a <starttie/> ... <endtie/> type construct would be necessary.  There might also need to be an id, since presumably one can have several tie bars in an equation, and one would need to match the starts and ends.

I'm interested in hearing if lack of tie bars in MathML has been an issue for others.


--Robert

Dr. Robert Miner
Vice President, Research and Development
 
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